Learn French: Le joueur court très rapidement. - The player runs very quickly.

To make an adverb like "quickly" in French, you usually take the feminine form of the adjective and add -ment. The adjective rapide (fast) already ends in an "e", so its masculine and feminine forms are exactly the same. We just attach -ment directly to get rapidement.

Could I just say "Le joueur court très vite" instead?

Yes, absolutely! Vite is an irregular adverb that also means "fast" or "quickly", and it is very common in everyday French. Rapidement means the exact same thing but sounds slightly more formal. Both are perfectly correct here.

Why are the adverbs très rapidement placed after the verb court?

In French, adverbs that describe how an action is done usually go right after the conjugated verb. While in English you might sometimes put the adverb before the verb (like "he quickly runs"), in French it must be court très rapidement.

I forgot, what is the infinitive form of court and how do I pronounce it?

The infinitive is courir (to run). It is an irregular verb. In the present tense for il/elle/on (like our player here), it is conjugated as court. Remember that the "t" at the very end of court is completely silent!