Breakdown of Hárið hans er farið að þynnast mikið.
vera
to be
mikið
a lot
fara
to go
að
to (infinitive marker)
Questions & Answers about Hárið hans er farið að þynnast mikið.
Why do we use farið here instead of farinn?
In the vera farinn að construction, the past participle of fara acts like an adjective and must agree with the subject. Because the subject hárið (the hair) is a neuter word, the participle takes the neuter singular form farið.
What is the significance of the -st ending on þynnast?
This is the middle voice form of the verb þynna (to thin something out). Adding the -st suffix makes it an intransitive process, meaning the hair is thinning on its own without a direct object. You will frequently see the middle voice used for natural changes of state like this.
Could we translate er farið að as 'is going to' instead of 'has started to'?
No, that is a common trap for English speakers! While the phrase fara að on its own can mean 'going to' do something, the specific aspectual structure vera farinn að (literally 'to be gone to') always indicates that an ongoing action or process has already begun.
Why does hans come after the noun, and why does hárið have the definite article?
When showing possession in everyday Icelandic, the possessive word almost always follows the noun, and that noun must take the attached definite article. So we say hárið hans (literally 'the hair his') rather than hans hár, which would sound unusually emphatic or poetic.