When describing the sounds of a language, there has always been two main tendencies:

The description of sounds in isolation (phonetics)

The descriptioon of sounds as part of a system (phonology)
Standardization in language teaching is also a current trend in both Europe and in the
USA.
As we will see in the Big Picture of Phonetics and Phonology, overemphasis has been placed upon the segmentation of sounds for analysis, which has influenced teaching of sounds in isolation, neglecting the real communicative application in speaking, listening and reading aloud. If we take into account the effect of the suprasegmental phoneme of stress, phonology can help us even when writing: contrary to what is commonly accepted, there is a close regular relation between spelling and pronunciation.
To understand this, you can observe the Phonetic System of English and the Big
Picture, taking you into phonology and phonemics (the sounds in the English language as a system), and then into phonics, the influence of contextual neighbouring sounds.
Phonics will show us there is a regular basis for establishing a rule of English pronunciation, which can be quite effective in the teaching and learning of English as a communicative system, through the four communicative skills, not only for speaking and listening but for reading (aloud) and writing.
In the XVth century, the suprasegmental phoneme of sentence stress and the neighbouring sounds produced a regular effect of lengthening of vowels in stressed open syllables and a shortening in stressed closed syllables, which influenced spelling.
Hence, we can deduce pronunciation from spelling. We can definitely extract the following pronunciation rule of English vowel sounds:
C’VC-(CV)
Stressed vowels in closed syllables are short and pronounced with a pure timber (similar to the 5 Spanish vowels): æ,e, ɪ,o,υ,
Examples: cat, lend, ton, fit, sum
C’V-(CV)
Stressed vowels in open syllables are lengthened, most into diphthongs: eɪ, ii, aɪ, əυ, jυ.
Examples: na·me, ‘e·ve·n·ing, no·se, fi·ne, ‘stu·dent
V Unstressed vowels are neutralized, mainly in /ə/, /-/, or near sounds (/
I
/)
Example:
‘com·for·t·able
* Some consonants (such as –r-, -v-, -ll, th, -m-, -n-, w-, clusters –lk, -ld, –nd, -mb, -st-) influence and change this, resulting in a different sound :
Examples: part, river, call, mother, walk, find, paste
* Affixes also change the perception natives have of cord stress and components, resulting in common patterns of stress and pronunciation (usually neutral sounds):
Examples: fam·ily, re·port, com·ic, con·fuse , un·known

Phonetics belongs to the Linguistic Sciences. It studies the production and percption of sounds in human communication. It is Phonology that considers these sounds in the language system, selecting and classifying them from the distinctive features they have in the language as a system. Here we can talk of phonemes:
Segmental
Vowels &
Diphthongs
Distintive features:
 Mouth opening

Place

Roundness

Length
Phonemes
Suprasegmental
Consonants
Stress
Rhythm
Intonation
Morphophonology
Morphophonology also plays an important role in the distinctive realization of vowels since stress determines the focus of the information unit. It is the phrase stress since a sentence is divided into tone groups, and thus the phrase is the the basic information unit. Remaining nonfocal words and syllables in a phrase are usually neutrilzed and centralized in schwa or short i. See the structural words such as the, can, are, some. The same applies at the level of words. Word stress determines the syllable whose vowel will be really distinctive. Satellite vowels are ne¡utrilized into schwa or short I too.
Affixation also helps determine the focal root syllable in a word that is usually stressed an distinctive.
However, some affixes makes this complex as they produce a stress shift (eer, -ical,) while others do not. (decision, decisive; history, historical.)
Phonics
Phonics or the contextual sounds and letters is the third aspect we can consider in determining the regular pronunciation of English words. The quality of the stress syllable