Learn Icelandic: Sonur þinn er alltaf glaður.

In Icelandic, possessive pronouns like minn (my) and þinn (your) almost always come after the noun they belong to. So instead of saying your son, Icelandic puts the possessive second and says son your (sonur þinn).

Does þinn change depending on the word it describes?

Yes! Because sonur is a masculine noun and it is the subject of the sentence, the possessive pronoun must take the masculine form, which is þinn. If you were talking about a feminine noun like a daughter (dóttir), you would use the feminine form þín.

Why does the adjective glaður (happy) end in -ur here?

Adjectives must match the gender of the noun they describe. Because sonur is a masculine noun, the adjective takes the standard masculine ending -ur. If the subject were a feminine noun, the ending would change.

Is there a trick to pronouncing the double L in alltaf?

Yes, in Icelandic, a double L is usually pronounced with a hard tl sound, similar to the end of the English word settle. So alltaf is pronounced a bit like ahtl-taf.