Ég sé bílinn hans fyrir framan húsið.

Bílinn is accusative singular definite. It is the direct object of sé, and the verb sjá normally takes an accusative object.

You can break it down like this:

• bíll = nominative singular, car
• bíl = accusative singular
• bílinn = accusative singular definite, the car

The ending -inn is the definite article attached to the noun. Icelandic usually adds the to the end of the word instead of using a separate word before it.

For example:

• bíll = a car
• bílinn = the car

With possessive forms like hans, hennar, and þeirra, the normal neutral position is after the noun:

• bíllinn hans = his car

Putting hans before the noun is not the usual default in everyday Icelandic. If it appears there, it usually sounds more emphatic or contrastive.

Sinn is a reflexive possessive, and it is used when a 3rd person subject owns something that belongs to itself. But the subject here is ég = I, so sinn cannot refer back to it.

Compare:

• Ég sé bílinn hans = I see his car
• Hann sér bílinn sinn = He sees his own car
• Hann sér bílinn hans = He sees his car, but the owner is some other male person

No. Hans stays the same. It does not agree with the noun the way some other possessive words do.

For example:

• bíllinn hans = his car
• bókin hans = his book
• húsið hans = his house

Historically, hans is the genitive form of hann, but as a possessive it is basically used as a fixed form.

Because that is a normal Icelandic pattern. English usually says his car, without the, but Icelandic often uses definite noun + following possessive for a specific item:

• bílinn hans
• bókin hennar
• húsið þeirra

So even if it looks word-for-word like the car his, it is perfectly natural Icelandic.

Because fyrir framan takes the accusative. The noun hús is neuter, and in the singular the nominative definite and accusative definite forms are both húsið, so the form does not visibly change here.

By contrast:

• húsið = nominative/accusative singular definite
• húsinu = dative singular definite

So húsið is the correct form after fyrir framan.

Yes, but the sentence as given has the most neutral word order:

• Ég sé bílinn hans fyrir framan húsið.

If you move another part to the front for emphasis, Icelandic still keeps the finite verb in second position:

• Fyrir framan húsið sé ég bílinn hans.

That second-position rule is a very important part of Icelandic sentence structure.