It can be difficult to talk about subjects like XXXX.
It can be difficult to learn a new language.
I hope it doesn't rain Today.
What does it means in the above sentences?
Why should we start the sentences with it?
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The first sentence could be rephrased as "Talking about subjects like XXXX can be difficult." while the second sentence could be rephrased as "Learning a new language can be difficult." The difference between the original sentences and the one I am suggesting is the word order. With the original sentences, the first words are "It can be difficult"; that could be done to put those words in evidence.
With sentences about weather, the verb (e.g. rain, snow) is impersonal, and intransitive. Since in English the subject of a sentence is normally not implicit, it is used. (In English you don't normally write a sentence like "Is playing football with his friends." or "Rains.")