Learn German: Nimm die Tablette regelmäßig ein.

This is the informal singular command for nehmen (to take). To make a command for one person you are on a first-name basis with, you usually take the du form of the verb and drop the -st. Because the du form of this verb is du nimmst (it changes the 'e' to an 'i'), dropping the -st leaves us with nimm. Also notice that the word du is dropped entirely in commands!

Why is there an ein at the very end of the sentence?

The verb here isn't just nehmen, it is the separable verb einnehmen (to take in / to ingest). In German commands, just like in normal present tense sentences, the prefix (ein) splits off from the main verb and jumps to the very end of the sentence.

Why use einnehmen instead of just nehmen for taking a pill?

While English uses the same word "take" for picking up an object and for swallowing medicine, German specifically uses einnehmen for ingesting medication. If you just said Nimm die Tablette, it might sound like you are telling someone to simply pick the pill up with their fingers.

Is die Tablette in the accusative case here?