Learn Finnish: Näytä kirjastokorttisi virkailijalle. - Show your library card to the clerk.

You use the negative command word älä plus the verb's connegative stem. The great news is that the connegative stem is exactly the same as the positive command form! So, "Don't show" is simply Älä näytä.

Why is it näytä with one 't' when the dictionary form is näyttää?

The command form (imperative) uses the weak grade of the verb stem. You can easily find this by taking the "I" form of the verb (minä näytän) and dropping the -n. The strong double tt weakens to a single t.

Why doesn't the object kirjastokorttisi have an -n ending to show it's the object?

Commands are an exception to the normal object rules. When telling someone to do something, the object typically takes the basic form with no -n ending. Additionally, whenever you attach a possessive suffix like -si (your), it automatically replaces any -n ending that might have been there!

What does the -lle ending on virkailijalle do here?

It is the allative case, which translates to "to" or "onto" in English. When you show (näyttää) something, you show it to someone. So virkailija (clerk) becomes virkailijalle (to the clerk).