The length of a vowel is only shown by it's special letter or letter-combination and the diacritic used. There is no influence from the surrounding consonants.
We have short vowels: a, e, i, o, u, ä, ö, ü, ó*, æ
long vowels: ā, ē, ī, ō, ū, ææ, öö, üü, óó, ie, oe, ue, äe, üe, öe (letters that carry diacritics other than the macron (line above the letter) have to be doubled to mark lengthening)
and "stressed" vowels: â, ê, î, ô, û, ææe, ööe, üüe.
For e, i, o, u, ö, ü there are two distinct phonemes in existance that make it necessary to distinguish between the long vowels ē/äe, ī/ie, ō/oe, ū/ue , öö/öe, üü/üe.
The letter e stands for short e as well as for the "Schwa". This is the reason why it is a component of some letter-combinations. The long vowels ie, oe, ue, öe, üe often sound as if they end in "Schwa".
The "stressed" vowels have to be spoken as a long rising and then short falling sound. The last, falling part often sounds "Schwa"-like.
* for ó also å can be used alternatively. Sometimes an ë is needed to make clear if none of the above mentioned combinations is meant.
Simple diphthongs are written with simple letter-combinations like ai, au, ei, oi, äi, öy.
Long diphthongs mostly follow the rules given for vowels or are marked by changing i to j: aj, oj, äj etc.
Most "stressed" diphthongs are written in letter-combinations with e like äie, öye or like âu.
All consonants are written as single letters except when connecting syllables (belonging to both of them) or in case of a prolonged consonant.
The consonants are: b, d, f, g, ğ, h, j, k, l, m, n, p, r, s, t, w.
ğ is the g-fricative (alternatively written gh). c only occurs in the combination ch (the sound like that in Scottish “Loch”). v is used to mark an indifferent sound between f and w. s means both the voiced and the voiceless sound as well (if there is a reason to distinguish them definitely the z is used for the voiced sound).