Wa(ha) 「は」 and ga 「が」particle usage
The particles は and が commonly appear in the Japanese sentences. Both mark a subject. To the untrained ears of students of Japanese language, it may not alarm them even if the use of particles are swapped in the sentences.
Although many Japanese may not be able to explain precisely, we are guessing, the reasons or conditions of appropriate use of these particles,
their ears have no problem of catching the oddity of sentences. They are distinguished and precisely used in Japanese sentences.
The particles は [wa] and が[ga] are perhaps the most troublesome and confusing particles for the students to learn. We would like to introduce a simple rule that enable you to use them more appropriately in most of time.
1) わたしが その本を買った。I (am the one who) bought the book.
2) わたしは その本を買った。(As for me,) I bought the book.
Quick Rule; 「が」emphasizes what preceeds the particle, and 「は」emphasizes what follows it. In the example 1), it emphasizes who bought the book. On other hand, in 2) it emhpasizes the fact of buying.
This helps you use them correctly most of times. Once you got used to this rule, you might read other more refined rules. Some suggest, in addition, 「が」in new information and 「は」in old or known information communication.
助詞「は」と「が」の使い分け (https://nihon5-bunka.net/usage-particle-ha-ga/)
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