"Grandfather will come in April."

Translation: दादा अप्रैल में आयेंगे ।

[AdamLynch2]

Can you put the अप्रैल में in the beginning of the sentence?

[vinay92]

Yes but it changes the subject of the sentence from the grandfather to April.

दादा अप्रैल में आयेंगे । is the answer to the question दादा कब आएंगे?

अप्रैल में दादा आयेंगे । is the answer to the question अप्रैल में कौन आएंगे?
But they are mostly interchangeable.