Learn Icelandic: Manstu hvernig þú notar stefnuljósið? - Do you remember how you use the turn signal?

The verb here is muna (to remember), and its "you" form is manst. When you attach þú to the end of a verb to form a question, it usually becomes -ðu. However, because manst ends in a hard -st sound, the suffix changes to -tu to make it easier to pronounce. So manst + þú becomes manstu.

Why do we use the full pronoun þú later in the sentence instead of attaching it to notar?

We only attach þú to the end of a verb in direct questions (like manstu) or commands, where the verb comes first. The second part of the sentence (hvernig þú notar...) is an indirect question. Here, the word order goes back to normal (subject then verb), so we use the separate pronoun þú before the verb notar.

What case is stefnuljósið in here?

It is in the accusative case because it is the direct object of the verb nota (to use). Because stefnuljós (turn signal) is a neuter noun, its accusative form looks exactly the same as its nominative dictionary form. The -ið at the end is simply the attached definite article meaning "the".

What are the parts that make up the word stefnuljósið?