'I hope it rains' or 'I hope it will rain'

I've been through several books for teaching English as a foreign language, and these structures are usually presented as exact synonyms. But isn't there a difference?

Just by looking at the verb tense, I'd say that using Present Simple gives the idea that the speaker thinks it is likely to rain, whereas using will places the possibility of rain in the realm of 'unlikeliness'.

Am I on to something here or am I just splitting hairs? Honestly, I feel awkward telling my students 'yeah, they're interchangeable just like the book says' when I'm not convinced.

English has no future tense, but expresses the future in various other ways. One of them is will + plain form of the verb (‘I hope it will rain’) and another is the present tense (‘I hope it rains’). Both are grammatical, but the second is what most native speakers will say in most contexts.

While I agree that the two sentences are mostly synonymous, I believe we are more likely to use "I hope it will rain" if it is a response to a sentence like, "Peter said it won't rain" because users might find it comfortable to pair "will" and "won't". This also emphasizes a direct opposition to the first statement.