Learn English – What rules of English allow the first t in “patient” to make an sh sound

I'd say that your German colleagues are mishearing the English pronunciations.

The German letter ü makes the sound [y], which does not occur in English.

The words loose, poodle, food, and most other words with oo have the vowel [u], which is usually spelled u or uh in German. Historically this is a long /o/ sound that was written with "oo", the pronunciation of which has shifted to [u] as a result of the Great Vowel Shift.

Some words with oo have instead the vowel [ʊ]: good, hood, book. There is no rule that predicts which words have this pronunciation, so you have to memorize it. The [ʊ] sound occurs in German as an allophone of /u/ in closed syllables. The vowel [ʊ] is shorter, more lax, and slightly centralized relative to [u]. This sound also tends to come from an older long /o/, though the reasons for this split are complicated and obscure.

A very small number of words with oo are pronounced with an [o] vowel: door, floor. These words always end in r, because the final r colors the preceding vowel. This is the same sound that is spelled o or oh in German.

Some tips for the production of schwa:

(1) Stand in front of a mirror. Don't close your teeth or open your mouth, just relax your face. Make a sound as if it's coming from your throat or chest (in reality it will be coming from your vocal folds). This should be a schwa sound. In the mirror you should not be able to see your face move at all. If you recorded a video of you practising schwa, but with no sound, we would not know when you were making a sound and when you were silent, because your tongue, jaw and lips - and your face in general - should all be relaxed and not moving at all.

(2) Try to make the sound /b/ as in the word big, but just /b/ on its own. Now try the sound /d/ as in dog and then the sound /g/ as in girl. Do this two or three times. When we say these sounds on their own, we automatically put a little vowel on the end - we have to because they are voiced. If you said the sounds correctly, then you probably said /bə/, /də/ and /gə/. The little vowel that you made by accident after the consonant is a schwa. This is because you were not trying to make any special vowel there.

Schwa only occurs in unstressed syllables. The reason we make schwa like this is because we need to make unstressed syllables shorter than other ones in English. We need the other stressed syllables to be longer and to stand out. Schwas are very quick to make because we do not need to move any of the articulators (the parts of our mouth that we use to make consonants or change the sounds of vowels). If we make a big articulation, a big movement of our mouths, like we do for /æ/ in cat, we need to move our articulators a long way. For /æ/, for example, we have to spread our lips very wide, and drop our jaw very low and move the 'front' (that means the middle) of our tongue so it raises slightly up towards the roof of our mouth. This all takes a lot of time. Because of this, /æ/ is actually quite a long sound, even though it belongs to the so-called 'short vowels'. For a schwa you do not need to move anything! In conclusion then, what you need to do to make a good schwa sound is: nothing!