Learn Hardcore Latin: Mater non sinit puellam extra ostium currere, quia pluvia cadit. - Mother does not allow the girl to run outside the door, because rain is falling.

Breakdown of Mater non sinit puellam extra ostium currere, quia pluvia cadit.

Why is puellam in the accusative, not puella?

Because puellam is the object of sinit.

The verb sinere means to allow, to let, and in Latin it often takes:

So:

Literally, the structure is something like:

The mother does not allow the girl to run.

That is why puellam is accusative.

Why is currere an infinitive?

Because after sinit, Latin normally uses an infinitive to say what someone is allowed or not allowed to do.

So:

This is a very common Latin pattern:

verb of ordering / allowing / wanting + accusative person + infinitive

In English we often translate it with to:

But Latin does not use a separate word for to here. The infinitive ending itself already carries that meaning.