Learn Portuguese (Portugal): Se te assustas com o fantasma, usas o amuleto. - If you get scared by the ghost, you wear the amulet.

The word Se (if) is a subordinating conjunction. In Portuguese, these types of words act like magnets that pull the pronoun to the front of the verb (a rule called proclisis). That is why we must say Se te assustas instead of Se assustas-te.

Why is it o fantasma instead of a fantasma if the word ends in an -a?

Even though it ends in -a, fantasma is a masculine word, which is why it uses the masculine article o. Many Portuguese words that end in -ma are masculine exceptions you just have to memorize, such as o problema, o poema, and o sistema.

Why do we need the pronoun te at all? Can I just say Se assustas?

The base verb assustar means to scare someone else. To say to get scared, Portuguese uses the reflexive form assustar-se (literally, to scare oneself). Since this sentence is about you (tu) getting scared, we need the reflexive pronoun te.

Both verbs (assustas and usas) are in the present tense. Could I use the future tense for the second part?