I could only help him when he was in trouble yesterday.
Is this sentence the same as: the only thing I could do was help him?
Not unambiguously. Your sentence could mean that the only time you could help him was yesterday. "Only" is tricky even for native speakers.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Not unambiguously. Your sentence could mean that the only time you could help him was yesterday.
"Only" is tricky even for native speakers. Its position in the sentence often seems to go against the intended meaning, but very often that doesn't matter because we can effortlessly tell what you mean. Your sentence, though correct, uses a rather old-fashioned definition of "only", at least