Learn German: Ich habe meinen Cousin lange nicht gesehen. - I have not seen my cousin for a long time.

Cousin is the direct object of the verb gesehen, so it must be in the accusative case. Because Cousin is a masculine noun (der Cousin), the possessive word mein changes to meinen.

Why isn't there a word for "for" in the German sentence?

While English uses the phrase "for a long time", German simply uses the adverb lange. Adding a literal translation like für would sound unnatural here.

Why does nicht come after lange?

In German, the word nicht usually comes right before the specific thing it is negating. Here, it negates the action of seeing (gesehen), so it is placed right before that verb at the end of the sentence. General time expressions like lange naturally come earlier in the sentence.

Could I use seit langem instead of lange in this sentence?

Yes, you can! Ich habe meinen Cousin seit langem nicht gesehen is also perfectly correct and means the same thing. However, lange is a shorter and very common alternative in everyday conversational German.