Sometimes in German grammar we describe certain elements as strong or weak to mark out how they behave.
Weak verbs
A weak verb is basically a regular verb; it’s ‘weak’ because it allows itself to be fully and completely ruled by the laws and prescriptions of fascistic regular verb behaviour, like a sad little peasant going to work in the fields with the rest of the oppressed masses. ‘Spielen’, ‘kochen’, ‘kaufen’ and ‘machen’ are all weak verbs.
Strong verbs
Strong verbs are ‘strong’ because they do their own thing; they break out from the shackles of regularity and change their vowels or entire root around depending on the person, the number and the tense they are in. Take a look at this madness:
essen – to eat
Present tense du form: du isst
Simple past tense er form: er aß
Conditional er form: er äße (this form is antiquated and almost never used, probably because it sounds so stupid when you say it out loud)
There are quite a lot of strong verbs (bleiben, fliegen, sehen, gehen) but they tend to settle into patterns where several verbs all do the same vowel changes. One of these days I’ll give you guys a list of good ones to learn.
Weak nouns
Quite a few masculine nouns are ‘weak’, meaning that they are pathetic and can’t hold their own in any other case than the nominative singular. If they are in any case other than the nominative singular, they take an ‘-(e)n’ at the end of the noun:
der Affe -> mit dem Affen
der Name -> Ich gebe dir meinen Namen
der Präsident -> das Hotelzimmer des Präsidenten
der Herr -> an den Herrn (here we just put an ‘n’ because ‘Herren’ has been shortened through time and casualness)
The most important thing to remember is this: men are weak. Yes.
**WARNING** There is one neuter noun which is also weak for reasons that nobody knows. Das Herz. Yes, the heart is a weak neuter noun, the only one which exists. Even crazier is that as well as adding an -en in all other cases than nominative, it also adds an -s on top of the -en in the genitive: des Herzens. Wild, eh?