"They do not write letters usually."
Translation:Ellos no suelen escribir cartas.
suelen is the verb. "soler" = "to be in the habit of"' and is often followed by an infinitive. "suelen" is 3rd person plural, so the sentence is "They are not in the habit of writing letters." I am not to keen on Duo's translation. "soler" i sused in the past tense to express the idea of "used to do" something