One of the most important things a non-native speaker must learn is that English has two types of vowels, strong and weak, which are closely related to the presence or absence of stress. But first I need to make it clear that the concept of weak and strong vowel in English has nothing to do with the categories of vocal fuerte and vocal débil in Spanish, it’s a completely different approach. And, of course, when I refer to vowels I’m talking about sounds (or, more precisely, phonemes), not letters. If I want to mention a letter, I’ll say it explicitly.
We’ll start by looking at the weak vowels, since it’s in this area where the differences between English and Spanish are bigger.
The commonest weak vowel is the schwa /ə/, which is the most frequent sound in English. Two crucial features must be remembered about the schwa:
- It only occurs in unstressed syllables.
- It can be spelled as almost any vowel letter or combination of vowel letters.
For example, the phoneme in the first syllable of the word about -which bears no stress- is not pronounced as an /a/, as many Spanish speakers tend to think, but as a schwa /əˈbaʊt/ 
.
Listen to how the first a in the word accession sounds and notice how different it is from the a in abbey, which is stressed and carries a strong vowel:
Seventeen months after the Queen’s accession, Vaughn Williams and Ursula took their places in the abbey (Donald Mcleod, BBC3).
On the other hand, in the second syllable of the word gorgeous, which is also unstressed, we pronounce a schwa as well. /ˈɡɔːdʒəs/
Listen to the word gorgeous in this presentation from The National Gallery:
Constable’s very fond of adding a red accent to balance out this gorgeous green (Emily Green, The National Gallery podcast).
As you can observe, the vowel sounds in these unstressed syllables are exactly the same, in spite of the fact that you have just one letter (a) in about and accession and three letters (eou) in gorgeous.
How can just a single vowel letter sound the same as a group of three vowel letters? This is a really surprising fact for a non-native speaker, but the sooner you get used to it, the better. The reason is that vowels in unstressed syllables tend to be weakened or, using another term, reduced. This means several things. First, they are produced with less movement of the articulators. The schwa, for example, is produced at the very centre of the mouth, with the tongue completely relaxed, and is neither rounded nor spread. Weak vowels are also shorter and softer. The sound is less distinct. Phoneticians talk about obscuration of the vowel, a word which conveys a good image of its quality. This is extraordinarily important for Spanish speakers, because our tendency is to produce the sound we identify with the letter (the a as /a/, the i as /i/, etc.).
From this we can draw a first conclusion: whenever a syllable is not stressed, there is a big chance that the vowel or vowels it contains will be pronounced as a schwa, as happens in a great majority of cases, or as another weak vowel. This is what we’ll see now.
Which phonemes can be weak? We’ve already said that the quintessential weak vowel is the schwa, but it’s not the only one.
The phoneme /ɪ/ can be both weak and strong. In words like women /ˈwɪmɪn/, minute /ˈmɪnɪt/, ticket /ˈtɪkɪt/ or biscuit /ˈbɪskɪt/, the first /ɪ/ is strong and the second weak.
In captain /ˈkæptɪn/ and manage /ˈmænɪdʒ/, the /ɪ/ is weak too, as is in may function words, such as it /ɪt/, this /ðɪs/ is /ɪz/, etc.
The capacity for functioning as both weak and strong vowels also applies to the phoneme /ʊ/, but the cases of weak /ʊ/ are not so common, mainly because this phoneme tends to be reduced to a schwa in unstressed syllables. We find a weak /ʊ/ in the word adulthood /ˈædʌlthʊd/, for example, and if you say beautiful as /ˈbjuːtɪfʊl/, you are using a weak /ʊ/ as well, although this word is most commonly pronounced in its further reduced forms of /ˈbjuːtəfəl/ or /ˈbjuːtəfl/ (the last option is a syllabic consonant, article coming soon).
Finally, there are other two weak sounds, the short /i/ and /u/ (they are not phonemes, but allophones, as explained here).
The short /i/ is extremely common, especially at the end of words: city /ˈsɪti/, country /ˈkʌntri/, basically /ˈbeɪsɪkli/, etc.
It can also be found in the middle of words, mainly before stresses (association /əˌsəʊsiˈeɪʃn/, geography /dʒiˈɒɡrəfi/) and in the article the before vowels, e.g., the accident ǀ ði ˈæksɪdənt ǀ.
The short /u/ is the sound we use in the particle to before vowels ǀ tu ʌs ǀ and also in words such as situation /ˌsɪtʃuˈeɪʃən/ or annual /ˈænjuəl/.
Have a look at this example, where the predominance of the schwa is very clearly illustrated.
There was a woman that had brought her daughter with her. ǀ ðə wəz ə ˈwʊmən ðət əd ˈbrɔːt ə ˈdɔːtə wɪð ə ǀ
In this simple sentence we have thirteen vowel phonemes, of which just three are strong vowels (/ʊ/, /ɔː/, /ɔː/). The remaining ten are weak vowels, nine of them schwas (/ə/) and only one an /ɪ/, which is weak because it’s placed in an unstressed syllable.
Said by a native speaker, this apparently harmless statement can pose a few challenges to Spanish or other non-native speakers. Let’s see them in detail.
- When the word there is used to announce the existence of something, it’s always unstressed and pronounced with a schwa /ðə/. It’s called the “existential there” and is the equivalent of hay, hubo, habrá, etc. in Spanish.
- The letter r is not pronounced in the words there, daughter and her because I speak British English, a non-rhotic variety, which means that the r is only pronounced when it’s followed by a vowel.
- The word was is normally pronounced /wəz/. There is a strong form /wɒz/, but this is just used for emphasis, contrast or at the end of a clause (and please, never say /waz/, as many Spanish speakers do).
- Here the word that is pronounced with a schwa (/ðət/) because it’s a conjunction. The strong form (/ðæt/) is used when it’s a demonstrative (that man).
- The auxiliary verb have is normally unstressed and pronounced with a schwa. The /h/ is very frequently dropped because of elision. Remember that these three features (unstressed, schwa and elision of /h/) only apply to the auxiliary. When have is a main verb with the meaning of tener, it’s stressed and pronounced in its strong form /hæv/.
- In the pronoun her, repeated twice in the sentence, the /h/ is dropped because of elision as well.
- There could be another complication in this sentence. In the phrase that had brought, the speaker could have used an assimilation of d to b. It is a very common process and it would sound like this . ǀ ðət əb ˈbrɔːt ǀ.
This is the type of analysis I do with my students in my one-to-one classes. I make them practise these processes with exercises until they improve their comprehension of native speakers and are capable of speaking like that themselves. If you are interested in my classes, you can contact me here.