When you want to give advice or a direct instruction in German — telling a friend ‘drink water!’ or a doctor telling a patient ‘take your medicine!’ — you need a special form called the Imperativ, the command form. This form changes depending on who you're speaking to: a close friend (du), or someone you don't know well or want to address respectfully (Sie).
With du, the basic rule is simple: take the verb stem without -en from the infinitive and put it alone at the start of the sentence, with no ending: gehen becomes Geh!, trinken becomes Trink! But watch out — verbs that change their vowel in the du-form of the present tense, like essen (du isst) or sprechen (du sprichst), keep that same vowel change in the imperative too: Iss!, not Esse!, and Sprich!, not Spreche! This is a small but important detail worth memorizing carefully.
With Sie, it's much easier: the verb stays in its full form exactly as in the present tense, and you simply add Sie right after it: Gehen Sie! Trinken Sie! Nehmen Sie die Tabletten!
Alongside the direct imperative, German has two gentler ways to give advice that come up constantly: sollten (‘should’), for soft, non-binding suggestions, as in Du solltest mehr Wasser trinken (‘you should drink more water’), and müssen (‘must’), for genuine necessity, as in Sie müssen zum Arzt gehen (‘you must go to the doctor’). Notice the difference in tone: sollten feels like advice from a friend, while müssen feels like a serious medical warning that leaves no room for hesitation.
In many languages the command form changes depending on the gender and number of the person addressed. German is simpler here — it makes no distinction at all between masculine and feminine in the imperative, only between du for one person informally, Sie for formal address (singular or plural), and ihr for a group addressed informally.
A common mistake is forgetting the vowel change in verbs like essen, sprechen, or nehmen when forming the du-imperative, producing Nehme die Tabletten! instead of the correct Nimm die Tabletten!