Learn French: Si tu es malade, tu ne travailles pas. - If you are sick, you do not work.

In this sentence, si means "if". You might also see si used to mean "yes" when someone is contradicting a negative statement (like saying "Yes I am!" to "You aren't going?"). But when it sets up a condition like in this sentence, it always means "if".

Why doesn't the word malade change depending on whether the person being spoken to is male or female?

The adjective malade already ends with an "e" in its default masculine dictionary form. In French, when an adjective already ends in "e", you don't add another "e" for the feminine form. It stays exactly the same!

How does the "do not" work in tu ne travailles pas?

To make a sentence negative in French, you frame the verb with two words. The ne goes right before the action word (travailles) and the pas goes right after it. Together, they mean "do not".

Do all verbs for tu end in "s" like es and travailles?

Almost always! When you use the informal "you" (tu), the conjugated verb usually gets an "s" at the end. We see it here in both the irregular verb tu es (you are) and the regular verb tu travailles (you work). Just remember that this final "s" is silent when you speak.