Apprendre is only the infinitive form of the verb to learn, and since we have extensive conjugations for the verbs, you have to conjugate and say:

J’apprends le français.

J’apprendre le français.

Is incorrect.

Apprendre is the infinitive verb so if you think of it as how the verb was born in to the world. apprends is conjugated to je so it would be j’apprends le francais
here are the present conjugations for apprendre.

Apprendre

j'apprends
tu apprends
il/elle/on apprend
nous apprenons
vous apprenez
ils/elles apprennent

The infinitive form in English (as given to foreigners learning it at least) is usually presented as “to X”, as in “to learn”. And it does correspond a lot to how it’s used in sentences – for example, you wouldn’t say “I to learn English” and you wouldn’t say “J’apprendre le Français”, but you would say “I want to learn English”, as you would say “je veux apprendre le Français”.

So in trying to figure out where you use the infinitive vs. a conjugated form of the verb, looking at where you’d use “to verb” in English would be a decent starting point.