Learn French: J'ai mal à la main.

In French, when you talk about body parts, you usually use a definite article like le, la, or les instead of a possessive word (like my or your). Because you already said J'ai (I have), it is obvious whose hand is hurting!

How does the phrase avoir mal à work?

The expression avoir mal à literally means 'to have pain in' or 'to have bad at'. You use the conjugated verb avoir (to have), add mal à, and then name the body part to say that it hurts.

Would I need to change à la if a different body part was hurting?

Yes. Main is a feminine word, so it uses à la. If the body part is masculine, like le dos (the back), à and le combine into au, making it J'ai mal au dos. For plural body parts, à and les combine into aux.

How could I use this sentence with parce que (because)?

Since you are learning how to give reasons, this sentence makes a perfect excuse! If someone asks Pourquoi tu n'écris pas ? (Why aren't you writing?), you simply put parce que in front of this sentence: Parce que j'ai mal à la main (Because my hand hurts).