If the information you are trying to say is common sense or a well-known thing, you should use "は". For example, ｢ロシアは広(ひろ)い。｣: "Russia is large." Everyone knows that Russia is a large county. So in this case, the particle is going to be "は".
On the other hand, if the information you are trying to say is new for listeners, you should use "が". For example, ｢私(わたし)のピロシキが盗(ぬす)まれた。｣: "My pirozhki has stolen." In this case, the information that the speaker trying to say is new for the listener (or the speaker is regarding so). So the particle is going to be "が".
I think it's similar to the relationship between the definite article and the indefinite article in English grammar. I don't know if it helps though 😅
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