Learn Farsi: احتمالا در خانه است.

The base word is the noun ehtemâl, which means probability. By adding the Arabic suffix -an, it turns into the adverb ehtemâlan, meaning probably. You will see this suffix added to many Arabic loanwords in Farsi to create adverbs.

Why does احتمالا end with the letter alef but get pronounced with an 'an' sound?

This uses a special Arabic spelling rule. The double dash symbol that sometimes goes on top of the alef is called tanvin and tells us to pronounce the ending as -an. In everyday typing, Iranians often skip the double dashes, so you just see a regular alef at the end, but you still pronounce it as ehtemâlan.

Where is the word for he in this sentence?

Farsi often drops subject pronouns like u (he or she) because the verb already tells us the person. The verb ast means he is, she is, or it is. You just rely on the context of the conversation to know exactly who is being talked about.

How would you say dar khâne ast in everyday spoken Farsi?

In casual speech, you will usually hear tu khune ast or tu khunast. The formal preposition dar (in) is usually replaced by tu, and the noun khâne changes pronunciation to khune.