Learn Turkish: Doktor tansiyonum için tuz kullanmamı yasakladı. - The doctor forbade me from using salt for my blood pressure.

It uses the short verbal noun structure you are currently learning: kullan (use) + ma (the act of using) + m (my) + ı (accusative case). Literally, the sentence says 'The doctor forbade my using salt.'

Is the -ma in kullanmamı the negative suffix?

No, in this case -ma is the short verbal noun suffix, turning the verb into a noun. If it were negative ('my not using'), it would have two -ma suffixes: kullanmamamı. The English translation uses 'from using' because the main verb yasakladı (forbade) naturally implies stopping an action.

Why is there an accusative -ı at the end of kullanmamı?

Because 'my using' (kullanmam) is the direct object of the verb yasakladı (forbade). Whenever you use verbs like forbid, want, or suggest with a specific action, that action noun needs the accusative case to show it is the specific thing being forbidden, wanted, or suggested.

Why doesn't tuz (salt) have any case suffixes?

Because it acts as an indefinite direct object for the verbal noun kullanmamı. When you talk about using salt in a general, non-specific sense, the word tuz stays bare and is placed immediately before the verb or verbal noun it belongs to.