Learn French: Tu as gratté ton tatouage hier. - You scratched your tattoo yesterday.

While we use reflexive verbs and definite articles for natural body parts (like Tu t'es gratté le bras for "You scratched your arm"), a tattoo is considered an addition to your body. You treat it like a regular object, using the non-reflexive verb gratter and the possessive adjective ton.

How could I use the grammar from this lesson to tell someone, "It is better not to scratch your tattoo"?

You would say: Il vaut mieux ne pas gratter ton tatouage. Notice that because gratter is in its unconjugated infinitive form here, the negative words ne and pas do not surround the verb—instead, they stay grouped together directly in front of it!

How can I remember that it's ton tatouage and not ta tatouage?

Tatouage is masculine, which is why it uses ton. A great rule of thumb for B1 learners is that almost all French nouns ending in -age are masculine (like le fromage, le voyage, and le message). There are only a few common exceptions, like la plage (the beach) or une image (a picture).

Could I put hier in the middle of the verb, like Tu as hier gratté?