Learn Portuguese (Portugal): Acho que vai chover amanhã. - I think it is going to rain tomorrow.

Both mean to think, but they are used differently. Achar is used when you are giving an opinion, making a guess, or expressing a belief, like in this sentence. Pensar is used for the actual brain process of thinking, like when you are thinking about a math problem.

In English, we often say 'I think it is going to rain' without the word 'that'. Can I drop que in Portuguese?

No, you cannot drop it. In Portuguese, the word que (meaning 'that') is mandatory to connect the two parts of the sentence. You must always say Acho que when expressing an opinion this way.

Why is it just vai chover without a pronoun for 'it'?

In Portuguese, verbs about weather like chover (to rain) are impersonal. This means they never take a subject pronoun like 'it' or ele. You simply use the third-person singular form of the verb on its own.

Could I use the future tense here instead of vai chover?

Yes, you could use the simple future tense and say choverá. However, using the verb ir (to go) plus the infinitive, like vai chover, is much more common in everyday spoken Portuguese, just like saying 'going to rain' in English.