How to chose between the strong and weak adjective inflection
I don't think German adjective inflection is that difficult theoreticly. But it is still easy to make mistakes of course.
The general rule is if you have an inflected determinant before the adjective you use the weak declination. If you have an uninflected determinant or no determinant at all you use the strong declination.
The same thing explained differently: If no word before the adjective signalises case, gender and number with a visible ending the adjective needs to do it = strong declination. If there is a word that signalises case, gender and number than we get the weak declination.
Common determinants are: dieser jener jeder aller solcher welcher, the definite and indefinite artikel, the possessive pronouns and kein
Learn these by heart.
The words dieser to welcher are usually always inflected. The definite artikel is always inflected. The rest of the words I listed can be inflected or not.
Er liegt auf dem roten Bett. In this case 'dem' is an inflected determinant that signalises case number and gender (dative neutre singular). That gives us the weak ending.
The weak endings are pretty much the same if you look at a chart of them. So they signalise case, number and gender weaker than the strong ones do, hence the names strong and weak. I hope you get what I mean.
Am Morgen trinke ich gern starken Tee. In this case there is no determinant. Thus we get the strong declination that better show us the gender, case and number. Here masculine singular accusative.
Gestern war ein sehr schöner Tag. Here we have a determinant (ein) but it is uninflected. That gives us the strong inflection. masc, nom, sg.
The determinants that can be both inflected an uninflected are uninflected only in masculine nominative and neutre nom and acc, (both sg.)
Masculine Neutre Feminine Pl
Nom mein mein meine meine
Acc meinen mein meine meine
Dat meinem meinem meiner meinen
Gen meines meines meiner meiner
The same pattern goes for the indefinite artikel and for kein. Look it up on a chart if you wish.
After some words where you would expect the weak declinaton the strong is used. The two most important of these are viele and wenige.
Viele interessante Leute
Wenige interessante Leute