Hi Shiloh,
Great question. Short answer: your translation is fine; there is no actual infinitive “to be” in the original Latin.
For everyone’s benefit, here is the full sentence: Mater filium suum monet ut laetus sit.
Whenever ut is followed by a verb in the subjunctive (as opposed to ut meaning “like/as” in a simile), you can break the sentence into two clauses:
- Mater filium suum monet
- ut laetus sit
a/the mother…a/the son…her own…is warning/advising, warns/advises
in order that, so that…happy…he/she/it is
The flexible part is that “he is” in the subjunctive form needs to be able to express hypothetical, potential action. So not “he is” as in the state of happiness is actually achieved, but “he could be,” “he might be,” or “he may be.” Think of ut meaning “in order that” even if you don’t translate it that way. Why did mom warn her son? So that he would become (another subjunctive translation) happy. The act of warning was real—the state of becoming happy was what she hoped would happen as a result.
Man, I love the subjunctive. As you move forward, you’ll see why the present tense of the subjunctive is used. Sneak peek: if mom’s action was in the past, the subjunctive form would need to change to express that. “She warned her son so that (at the time she warned him—not necessarily now—he would be happy.”
Fun stuff!