彼女のお母さんは手紙を書いています。

she; her; her; girlfriend; sweetheart

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

mother

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

letter

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to write; to compose; to pen; to draw; to paint