Learn Finnish: Isä harmaantuu, sillä hänellä on yhä enemmän stressiä. - Father is turning grey, for he has more and more stress.

When yhä is paired with a comparative word like enemmän (more), it expresses continuous, ongoing change. In this structure, instead of meaning "still", it translates to "more and more" or "increasingly".

The verb harmaantuu comes from the color harmaa (grey). How does this verb type work?

The ending -ntua (or -ntyä) is often added to adjectives and nouns to create verbs that mean "to become [adjective/noun]". So harmaa (grey) becomes harmaantua (to turn grey).

Why use sillä here instead of koska?

Both mean "because" or "for", but sillä is a coordinating conjunction that is slightly more formal or literary. It translates very well to the English word "for" used as a conjunction. In everyday spoken Finnish, you would usually hear koska.

Why is stressi in the partitive form (stressiä)?

It takes the partitive form because it follows enemmän (more). Words that express an indefinite amount or quantity require the uncountable noun following them to be in the partitive.