Learn Hardcore Latin: Si stabulum bene clausum erit, nullus bos nocte exibit. - If the stable has been well closed, no ox will go out at night.

Breakdown of Si stabulum bene clausum erit, nullus bos nocte exibit.

Why does the si clause use clausum erit instead of a present tense?

Because Latin often uses the future perfect in an if clause when the action must be completed before the main future action happens.

So:

Si stabulum bene clausum erit, nullus bos nocte exibit.

literally means something like:

If the stable will have been well shut, no ox will go out at night.

In natural English, we usually say:

If the stable is well shut...
or
If the stable has been well shut...

So this is a very normal Latin pattern: future perfect in the si clause, and future in the main clause.

Is clausum erit one verb or two words?

It is a two-word verb form.

Together they form the future perfect passive indicative:

clausum erit = will have been shut / closed

So even though it is written as two words, it functions as one verbal idea.

Why is it clausum and not clausus or clausa?

Because clausum agrees with stabulum.