Learn French: Si tu veux éviter un accident, tu ne passeras pas sous une échelle. - If you want to avoid an accident, you will not pass under a ladder.

This sentence uses the first conditional to talk about a likely future result. It follows the pattern Si + present tense (tu veux) + future tense (tu ne passeras pas). Notice that we do not use the future tense right after si, just like in English where we say 'If you want' and not 'If you will want'.

Where do the negative words ne and pas go in the future tense here?

They go on either side of the conjugated verb, just like in the present tense! In tu ne passeras pas, the ne goes right before the future tense verb (passeras) and the pas goes right after it.

Why is éviter in its infinitive form (ending in -er) instead of being conjugated?

When you have two verbs back-to-back in French, the first one is conjugated (tu veux) and the second stays in the infinitive (éviter). It works just like English: 'you want' (conjugated) + 'to avoid' (infinitive).

I learned that we often drop the vowel when the next word starts with a vowel or silent 'h'. Why doesn't si drop its 'i' here?