"tiene un buen dia mañana"
Usually it is written in the imperative (or subjunctive) form.
[Espero] que tenga un buen día mañana.
Ten un buen día mañana.
These exhortations mean: Have a nice day tomorrow.
The present tense, indicative looks a little out of place there since we are discussing a future time (tomorrow).
It means have a nice day but it's worded a bit different here. Usually you hear (Que) tenga un buen día which is subjunctive. Subjunctive indicates a possibility. Maybe you will have good day tomorrow but I can't say for sure. Maybe you won't but I hope you will etc..
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Que tenga un buen día mañana. Have a good day tomorrow! You have to use the subjunctive. The (Ojalá / Espero - to name a few - is cut off from the beginning. for example: "Espero que tenga un buen día"), and what is left is "Que tenga un buen día".
(Espero,ojalá,deseo) que tenga un buen día mañana.
Verbs in the main clause that express a wish, a preference, a request, or similar desire set up the need for the subjunctive in the subordinate clause. In these situations, the subject tells what he or she would like to happen; whether it will actually happen is not certain.
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