Vowels

For speakers of English the consonants of Esperanto don't present much of a problem (with the exception of /r/ and /l/), because they are very similar to those of English.  The vowels are more difficult.  (Just the opposite is true for speakers of Spanish.)  Even so, the vowels of Esperanto are similar to those of English, and to pronounce them correctly it is sufficient to focus on diphthongization and vowel reduction.  Don't diphthongize Esperanto vowels and don't reduce them to schwa.

However, you may find a more complete description of the Esperanto vowels helpful and/or interesting.  Such a description follows.

The five vowels of Esperanto and their English counterparts
 

Vowels are produced when the lungs push air through the mouth.  The mouth functions as a resonance chamber, and by varying the shape of this chamber, primarily by means of the tongue, we produce different vowel sounds.  Consonants are characterized by a complete or partial blockage of the oral cavity, whereas in the case of vowels there is no blockage and we can prolong the vowel sound as long as we can continue to expel air from the lungs.  The vocal chords vibrate during vowel production.

Many languages have a basic vowel system consisting of five vowels, and Esperanto is one of them.  These five vowels can be represented in a 2-dimensional diagram based on the position of the tongue.

        front  central  back

 high    i               u         beet      boot       see    Sue

  mid    e               o         bait      boat       say     so

  low             a                    bot                father

The letters which represent these vowels in Esperanto appear in the diagram to the left.  These letters are used in the same way in the International Phonetic Alphabet and in many languages, e.g., Spanish.  English is probably unique among the languages of the earth in the way in which it uses these letters to represent vowel sounds.  This is a consequence of a major change in English pronunciation which occurred about 600 years ago and is called the Great English Vowel Shift.  The English words to the right indicate the sounds approximately.  Pronounce these sounds and try to feel what your tongue is doing.  It is also helpful to look in a mirror.  Note also the role of the lips.

There is an important difference between the non-low Esperanto vowels and their English counterparts.  All five vowels in Esperanto are pure in the sense that there is no tongue movement during their production.  That means that they can be prolonged as long as we continue to expel air from the lungs.  The non-low English vowels are diphthongs consisting of a pure vowel following by tongue movement.  This tongue movement is called a semi-vowel or an off-glide.  The term 'off-glide' is suggestive because the tongue either moves or glides upward and forward to the position of the high front vowel /i/ (a front off-glide indicated in Esperanto by the letter 'j') or upward and back to the position of the high back vowel /u/ (a back off-glide indicated in Esperanto by the letter 'ŭ').  You should be able to feel and/or see the movement of your tongue and your lips.  The English vowels could be indicated by the following diagram:

        front  central  back

  high    ij             uŭ         beet      boot       see    Sue

   mid    ej             oŭ         bait      boat       say     so

   low            a                      bot                father

Unlike English the five basic vowels in Esperanto are not diphthongs, and it is very important for English speakers to strive to avoid tongue movement and to produce pure vowels.  The vowels in Spanish are a good model.
 

Diphthongs

In addition to these 5 pure vowels, Esperanto has 7 diphthongs, only 3 of which are common.  They are

        front  central  back

  high                   uj                   buoy               gooey

  mid    ej              oj         bay       boy        say     soy

  low             aj                     buy                 sigh

        front  central  back

  high            

  mid    eŭ              oŭ                    boat                low

  low             aŭ                     bout                  cow

With the exception of /eŭ/, which does not occur in English, these diphthongs are pronounced as in English.  /eŭ/ occurs in Spanish in the word 'Europa' = Europe.  It is easy to produce.  Just start with the pure vowel /e/ and move the tongue upward and back to the position of /u/.

/aŭ/ as in English 'bout', /aj/ as in English /buy/, and /oj/ as in English 'boy' occur frequently in Esperanto and in other languages.  /eŭ/ and /uj/ are somewhat less frequent.  All five have something in common which is lacking in the 4 English diphthongs which are to be avoided.  The off-glides are intrinsically high, and /aj/ and /aŭ/ combine a low vowel and a high off-glide.  /oj/ and /uj/ combine a back vowel and a front off-glide, and /eŭ/ combines a front vowel and a back off-glide.  In each case there is contrast between the position of the vowel and the off-glide.  To be avoided are the cases where there is no contrast:  a front non-low vowel combined with a front off-glide, and a back non-low vowel combined with a back off-glide.

Lax vowels

The 4 non-low English vowels mentioned above with the off-glide removed are common in the languages of the world and are called long vowels or tense vowels.  In English there is a corresponding set of short or lax vowels.  The /a/ in 'father' is also a lax vowel.  In the production of tense vowels, the muscles of the mouth are relatively tense; in the production of the corresponding lax vowel, they are relaxed and the tongue tends to move slightly toward the center of the mouth.  Try pronouncing some of the tense/lax pairs and see if you can feel these changes.  For the lax vowels in the following chart I use the standard IPA  (International Phonetic Alphabet) symbols.

        front  central  back

  high    ɪ               ʊ         bit      put          sit     soot

   mid    ɛ               ɔ         bet      bought       set     saw

  low             a                    bot                  father

The lax vowels of 'bit', 'put', and 'bought' do not occur in Esperanto, and we needn't concern ourselves with them further.  However, both the tense vowel /e/ and the lax vowel /ɛ/  of 'bet' occur in the speech of many speakers of Esperanto, and the distribution probably depends on the distribution in the native language of the speaker.  Use whichever one makes you feel comfortable.  Both also occur in Spanish, where the distribution is as follows:  before /r/ or /rr/ and in closed syllables (those which end in a consonant) use [ɛ]; otherwise use the tense vowel [e].  In fact among native speakers there is substantial variation in this distribution.

Vowel reduction

One other important matter needs to be mentioned.  Another vowel is common in English.  It is the mid central low-energy vowel called schwa designated by the symbol /ə/.  I call it a low-energy vowel because the tongue is in a neutral middle-of-the-mouth position, and it doesn't take much effort to produce it.  Loosely speaking, lax vowels become schwa when they are not stressed.  This process is called vowel reduction.  An example is the pair galaxy/galactic.  In the first member of the pair, the second vowel is unstressed and is reduced to schwa.  In the second member, the first vowel is unstressed and is reduced to schwa.  Vowel reduction does not occur in Esperanto, and English speakers should strive assiduously to avoid it.  Every vowel has the same sound, whether stressed or unstressed.

On-glides
 

The glides mentioned above, which indicate tongue movement, are often called semi-vowels.  They always occur next to a pure vowel.  When they follow the vowel, they are called off-glides and convert the pure vowel into a diphthong.  They can also occur before a vowel, in which case they are called on-glides.  They still indicate tongue movement, but now in the opposite direction.  The front on-glide represented by the letter 'j' is movement of the tongue from the position of the high front vowel /i/ to the position of the following vowel, and the back on-glide represented by the letter 'ŭ' is movement from the position of the high back vowel 'u' to the position of the following vowel.

Linguists create models to describe languages.  These models are not absolute, and different models may be useful for different purposes.  In the above description I have suggested that Esperanto has 12 vowels, 5 of which happen to be pure vowels and 7 of which happen to be diphthongs.  It would also be possible to say that Esperanto has only 5 vowels along with 2 semi-vowels and that there are 7 combinations of vowel + semi-vowel, which we call diphthongs.  In this latter view not all possible combinations of vowel + semi-vowel occur.  It may be convenient to consider the on-glides as part of the system of consonants.

Consider the two words 'mielo' = honey and 'mjelo' = medulla spinalis, part of the nervous system contained in the spine.  In order to pronounce the two vowels in 'mielo' which occur in succession, the tongue has to move from the position of /i/ to the position of /e/.  That movement is precisely how I have described the on-glide /j/, and correspondingly the difference in pronunciation between the above two words is very slight.