Learn Icelandic: Er þessi blýantur þinn?

The word for 'this' changes to match the gender of the noun. Because blýantur (pencil) is a masculine noun, we use the masculine form þessi. If we were talking about a neuter noun, we would use þetta.

Why does the sentence start with the verb Er?

To ask a simple yes/no question in Icelandic, you move the verb to the very front of the sentence, exactly like in English. Er means 'Is', so the order is perfectly matched: Is (Er) this (þessi) pencil (blýantur) yours (þinn).

Does blýantur need the 'the' ending when used with þessi?

No, it doesn't. When you use the word þessi (this) right next to a noun, the noun stays in its standard base form without the attached 'the' ending. That is why it is just þessi blýantur and not þessi blýanturinn.

Why is the word for 'yours' written as þinn?

Words that show ownership must match the gender of the noun they describe. Since blýantur is a masculine noun, the word for 'yours' must take the masculine form, which is þinn.