'Avere mal di': how Italian talks about aches
When something hurts in Italian, you usually reach for a ready-made chunk: mal di followed by the body part, used with the verb avere. So a headache is mal di testa, a sore throat is mal di gola and toothache is mal di denti, and you say ho mal di testa, literally I have ache of head.
English speakers tend to trip over this in two ways. English says a headache, so learners add an article and say ho un mal di testa where Italians would simply say ho mal di testa. English also says my head hurts, so learners try la mia testa fa male, which sounds odd. Italian does without both the article and the possessive in this pattern.
The pattern has three fixed parts: a form of avere, the word mal (a shortened form of male, pain) and di plus the body part, with no article in between. Nothing changes except the verb, which agrees with the person who is in pain: ho, hai, ha, abbiamo, avete, hanno.
Notice that mal does not change for plural people either. Several children can all avere mal di gola, while English has to switch to sore throats. The rule of thumb: think of mal di testa as one block, like a single word, and put avere in front of it.
The same block works in other tenses too: ho avuto mal di testa tutto il giorno means I had a headache all day.
mal di testa is the one you will use and hear most. It covers any headache, from a mild one to a bad one.
mal di gola is a sore throat, very common in winter and in conversations with doctors and pharmacists.
mal di denti is toothache. Here the body part is plural, denti, even when only one tooth hurts. That is simply the fixed form of the expression.
The same chunk gives you other everyday expressions: mal di pancia (stomach ache, tummy ache), mal di stomaco (stomach ache), mal di schiena (backache), mal d'orecchi (earache) and mal di mare (seasickness). They all follow exactly the same pattern with avere.
The article is left out in the neutral phrase, but it can appear once you describe the ache with an adjective. Ho un forte mal di testa is perfectly correct and very common.
When mal di testa is the subject of the sentence, it behaves like any other noun and takes the article il.
If you want to say that a body part hurts, the natural alternative is fare male, which works like piacere: the body part is the subject and the person is an indirect object. This is the correct way to express the idea behind la mia testa fa male.
Finally, mal di only exists as a fixed chunk for a handful of body parts. For other parts, such as a knee or an arm, Italians normally use fare male or avere male a with the article: ho male al ginocchio, not ho mal di ginocchio.
For common aches, use avere plus the fixed block mal di plus the body part, with no article and no possessive: ho mal di testa, hanno mal di gola, ha mal di denti. Add un only when you describe the ache (un forte mal di testa), and switch to mi fa male when you want the body part itself as the subject.