Also, Niko texted that he will be absent today because he has a flu.
which one is correct ? has a flu or had a flu? also i have search about the flu and flu is different thing.
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I would say:
Also, Niko texted that he will be absent today because he has the flu.
the is used specifically with flu. flu is a general term used for influenza and sometimes other common illnesses.
If you use influenza, no article is used. I suppose that is because it is a specific disease name:
Also, Niko texted that he will be absent today because he has influenza.
Regardless of the a/the issue which user3169 has answered, had is past tense, while has is present tense.
"Niko cannot come to work today as he has a cold."
"Niko cannot come to work today as he has the flu."
"Niko could not come to work yesterday as he had a cold."
"Niko could not come to work yesterday as he had the flu."
Use "has a flu" or "has the flu" if he is sick right now, and use "had a flu" or "had the flu" if he was sick earlier but now he is not sick.
You can also say "he caught the flu" to mean that he became sick earlier (and is still sick now).
The original question contains reported speech ("Niko texted that"). In reported speech, you have to change the tense (present becomes past, simple past becomes past perfect, etc). Scott's and Tanner Swett's answer did not consider this. Because of the backshift, 'will be' normally becomes 'would be'.
There are cases when backshift is optional:
Therefore I would say (if Niko is still ill):
"Also, Niko texted that he will be absent today because he has the flu."
Edit: according to the above rule, the will->would backshift is optional here, too.