1. He never failed to keep his word.

Question

2. He never failed to keep the word. 3. He never failed to keep a word.
4. He made a word with her.
5. He had a word with her.
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I could see the pattern such as break one's word, break the word. However, I cannot see the expression, break a word. Can we use #3 or #4 containing 'a' before word? In this case, 'word' means 'promoise'.

Answer 1

1 and 5 are fine, but not the others. There is no substitute for either sentence.

Also, 1 and 5 have very different meanings.