Learn French: N'ouvre pas le cadenas avant de vérifier le chronomètre ! - Do not open the padlock before checking the timer!

Even though ouvrir (to open) is an -ir verb, it is conjugated like a regular -er verb in the present tense (tu ouvres). In French, command forms (the imperative) for the tu subject drop the final -s for all verbs ending in -es or -as. So, tu ouvres simply becomes ouvre ! (or n'ouvre pas ! in the negative).

Why do we say avant de vérifier instead of just avant vérifier?

When you want to say "before doing something" in French, you must use the phrase avant de followed by the infinitive (unconjugated) form of the verb. You cannot put avant directly next to an infinitive on its own.

Could I use avant que here instead of avant de?

Not in this exact sentence. You use avant de + infinitive because the person performing both actions is the same (you are the one not opening the padlock, and you are the one checking the timer). You would use avant que (which requires the subjunctive mood) only if the subjects were different, such as "Don't open the padlock before HE checks the timer."

Does the -s at the end of cadenas mean it's plural?