Learn Arabic (Modern Standard): سرحت الأم شعر ابنتها بمشط من خشب. - The mother styled her daughter's hair with a wooden comb.

The verb is feminine, so it ends with a feminine marker ت that usually takes a sukun (a silent pause). However, the next word الأم starts with a silent alif followed by another sukun on the letter lam. To avoid the grammatical problem of having two silent consonants back-to-back, Arabic adds a temporary kasra (an 'i' sound) to the ت. You read it as 'sarrahati-l-umm'.

Why does the sentence use من خشب (from wood) instead of a specific adjective for 'wooden'?

In Arabic, it is extremely common to describe what an object is made of by using the preposition من (from/of) followed by the noun for the material. While you could technically turn some materials into adjectives, using من is the standard and most natural way to express things like a 'wooden comb' (مشط من خشب) or a 'gold ring'.

Why doesn't the word for daughter (ابنتها) have a hamza on its first letter?

The words for son and daughter (ابنة) start with a hamzat wasl (a connecting alif). When this letter appears in the middle of a sentence, you drop the vowel sound completely and glide the pronunciation straight from the previous word into the ب. It is written without the little hamza symbol to show that it is smoothly connected, sounding like 'sha'ri-bnatihaa'.

The root is س-ر-ح. How does the doubled middle letter change the meaning of the verb سرحت here?

This is a Form II verb, which is marked by the shadda (doubling) on the middle root letter. Form II often takes a basic root and turns it into a causative or intensive action. While the Form I root relates to letting things roam or flow freely, the Form II verb سرح means to cause the hair to flow freely and without tangles, which translates to 'to comb' or 'to style'.