I did my homework and met an offer.
He felt tired because he had worked a lot.
why I have to use 'had' if I can say that:
He felt tired because he worked a lot.
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Sign up to join this communityI did my homework and met an offer.
He felt tired because he had worked a lot.
why I have to use 'had' if I can say that:
He felt tired because he worked a lot.
Both sentences are grammatical, but they are typically interpreted differently.
He felt tired because he had worked a lot.
This means that he was tired because of a particular event in the past. It's referring to a certain incident, or span of time, that made him tired. (Generally in contrast to him not being tired.)
He felt tired because he worked a lot.
This means that he was generally tired because he had a habit of working a lot. It's something that occurred regularly. Rather than being tired on a specific day after pulling an all-nighter, for instance (where the addition of had is more appropriate), he always worked a lot and was always tired.
Which tense you use is determined by which of those senses you are trying to convey.