Learn French: Ouvre la bouche, s'il te plaît.

In French commands (the imperative mood), you drop the final 's' for verbs ending in -er when talking to a tu. Even though ouvrir ends in -ir, it is conjugated exactly like an -er verb. So the present tense tu ouvres becomes the command ouvre.

The English translation says 'your mouth'. Why does the French say la bouche instead of ta bouche?

When referring to body parts, French usually uses definite articles like le, la, or les (the) instead of possessive adjectives like mon or ton (my/your). Because you are giving a command directly to the person, it is already completely clear whose mouth is opening, making ta unnecessary.

How can we tell this sentence is directed at someone we know well?

There are two clues that show the informal tu register is being used here. First, the verb is in the singular informal command form (ouvre instead of the formal ouvrez). Second, it uses the informal version of please, s'il te plaît, rather than the formal s'il vous plaît.

What is the literal breakdown of s'il te plaît?