It’s called Tiefschwa. It’s exactly the -er ending which we generally pronounce like the end of butter. We do so for words ending in -r, too, like wir, Bier, Meer, Friseur.
Words are always pronounced the same, in general. Depending on the position in the sentence or what the speaker likes to lay focus on it might appear to sound different as the word gets spoken faster or slightly mumbled.
There are two ways each vowel can be pronounced - long and short. And there are diphtongs like ei, eu, au etc…
Vowels are pronounced long if they are doubled as in leer (empty, void), followed by an h as in Fehler (error, mistake) or if they are emphasized.
Which brings me to
I have googled it as I was pretty sure there must be some rules, but as a native speaker I obviously never paid much attention to it. It actually seems to be a rather complicated and irregular mess and I even stumbled upon german science papers dealing with that topic
However, there are two good rules of thumb which I think should work most of the time.
- Usually the emphasis lies on the first syllable of the word stem.
- If there is a prefix, it will be the emphasized syllable if it is seperable, otherwise follow rule 1.
Emphasis is done both by raising the voice as well as increasing the length of the syllable, although in everyday speech we usually tend to more use the second thing.
To give an example of rule 2 from above. The word setzen has its emphasis on the first syllable. The word übersetzen has two different meanings, and only in one case the prefix is seperable.
- übersetzen with stress on the second syllable (inseperable prefix) which means to translate
- übersetzen with stress on the first syllable (seperable) which means to ferry across (in this case über corresponds to across)
I had to think of how those words sound spoken out clearly in order to verify this for myself, because in everyday life you will hardly hear any difference. The emphasis is often done in a rather subtle manner.
I actually always considered English and German phonetics to be quiet similar, only that we aren’t so erratic with the vowels. But maybe the intense exposure to English here in Germany just made me get used to it before I even started learning the language.
Nevermind, I hope the above explanations are somewhat of use.