"The quiet old man and the quiet woman are sleeping."
Translation:Senex tacitus et femina tacita dormiunt.
Uncertain what you're trying to tell me. This exercise is one of translating English to Latin. Both placidus and tacitus are capable of having many English translations. Tacitus leans towards the sense of 'not speaking', 'silent', which can be extended to 'quiet'. Placidus can mean placid, quiet, calm. Asked to translate the 'quiet old man', etc, 'placidus', I think, is a fair choice of adjective. I suppose what you're saying is that, simplistic though it be, you have to use the one pre-ordained choice of word. Tacitus = quiet so any other adjective wrong.
You only use sunt when you're expressing:
WHERE "they are": In forō sunt , They are in the forum.
WHO or WHAT "they are": Senātōrēs sunt , They are senators. (Verb "to be" + a predicate noun)
Laetī sunt , They are happy. (Verb "to be" + a predicate adjective)
Any time they are DOING something, you do not use sunt , since Latin doesn't use "helping verbs" the way English does.
Latin dormiunt = They sleep (simple present) and also = They are sleeping (present progressive) and also = They do sleep: The latter is the form we use in negatives (They don't sleep) and in questions (Do they sleep? They sleep, don't they?)
So, English has 3 ways to translate each Latin "action" verb (in the present tense).
Senēs is a plural: "the old men." The singular "old man" in the nominative is senex .
You have two separate verbs in your sentence: "they sleep they are." In Latin, dormiunt by itself = they sleep / they are sleeping / they do sleep.
Senex tacitus et fēmina tacita dormiunt is a complete Latin sentence.