Learn French: Tu manges un croissant ? - Are you eating a croissant?

In French, the easiest way to ask a yes/no question is to keep the exact same word order as a statement, but make the pitch of your voice go up at the end. In writing, the question mark is your only clue! French doesn't use helper words like 'do' or 'are' to make questions this way.

Does Tu manges mean 'You eat' or 'You are eating'?

It means both! French does not have a separate 'are eating' form. Tu manges translates to either 'You eat' (in general) or 'You are eating' (right now), depending on what makes sense in the conversation.

Why does manges have an 's' at the end if we don't pronounce it?

Regular French verbs that end in '-er' (like manger) always get an '-s' at the end when they are paired with tu (the informal word for 'you'). Even though it is completely silent when you speak, it is a strict rule for spelling!

Why do we use un here instead of une?

Every noun in French has a gender. The word croissant is masculine, so you must use the masculine word for 'a', which is un. If the food were feminine, like pomme (apple), you would use une.