Usage of は and が in 文明が進むと、社会は複雑さを増す


> 文明が進むと、社会は複雑さを増す。

In this sentence,

1 - What are the reasons for using が and は particles in their respective
places?

2 - Could we replace any of these two particles with the other one? If yes,
how would that change the meaning?




>   1. What are the reasons for using が and は particles in their respective
> places?
>

文明が進むと is a [subordinate clause](https://www.grammar-
monster.com/glossary/subordinate_clause.htm) (the と-clause) of this sentence.
Since 文明 is not the topic of the whole sentence but the subject of a
subordinate clause, が imust be used as a simple subject marker. On the other
hand, 社会は複雑さを増す is the main clause of this sentence. Since 社会 is the topic of
this whole sentence, は has to be used.

The rule of thumb is that you cannot usually use は in subordinate clauses,
which includes relative clauses, when-clauses, if-clauses, and so on. (As an
exception, a contrastive-wa may appear in subordinate clauses.)

> 2 - Could we replace any of these two particles with the other one?

No, you can't (unless an explicit contrastive meaning is clearly intended).




>   1. What are the reasons for using が and は particles in their respective
> places?
>

文明が進むと is a [subordinate clause](https://www.grammar-
monster.com/glossary/subordinate_clause.htm) (the と-clause) of this sentence.
Since 文明 is not the topic of the whole sentence but the subject of a
subordinate clause, が imust be used as a simple subject marker. On the other
hand, 社会は複雑さを増す is the main clause of this sentence. Since 社会 is the topic of
this whole sentence, は has to be used.

The rule of thumb is that you cannot usually use は in subordinate clauses,
which includes relative clauses, when-clauses, if-clauses, and so on. (As an
exception, a contrastive-wa may appear in subordinate clauses.)

> 2 - Could we replace any of these two particles with the other one?

No, you can't (unless an explicit contrastive meaning is clearly intended).

