Question

thick clouds have gathered in the sky it ____ rain this evening​

Answer 1

Answer:

thick clouds have gathered in the sky it _may_ rain this evening

Answer 2

Thick clouds have gathered in the sky it may rain this evening.  

Explanation:

• "May" is used in the sentence as the speaker analyzes the situation and predicts a strong possibility.
• "May" is a modal auxiliary verb. It is also used for permissions along with talking about the possibility as in this case.

• There are other auxiliary verbs such as can, will, might, could, should, shall but this question depicts a possibility, therefore, "may" is used.