'The way' and 'how' are not used together.
I know the way how he did it. (Incorrect).
I know the way he did it. (Correct)
Or,
I know how he did it. (Correct)
But
"I know the reason why he did it."
"I know the time when he did it."
"I know the place where he lives."
These are correct.
What is the grammatical rule?
Sandip Kumar These are correct. Debatable. " There is really nothing wrong with "why" there, but it often strikes many of us as redundant.
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Sandip KumarThese are correct.
Debatable.
You hear "reason why" a lot, but in formal writing and careful speech, use "reason that" or just "reason": "I know the reason he did it." There is really nothing wrong with "why" there, but it often strikes many of us as redundant.
I'm having trouble with "I know the time when he did it." If you mean tha
How is not a relative pronoun. The relative pronouns are who, that and which.
In the first sentence, way is the direct object and it is followed by a defining relative clause. The relative pronoun can be omitted.
I know the way ( that ) he did it.
I know the way in which he did it.
In the alternative the content clause
Sandip KumarWhat is the grammatical rule?
Simply put, "way how" is the exception in the paradigm. That's "the rule".
person who / person that
thing which / thing that
time when / time that
place where / place that
reason why / reason thatway how / way that
CJ