A: 息子（むすこ）は ケガをしたので 仕事（しごと）を 休（やす）んだ。
has quite a different meaning from
Both sentences have two verbs: ケガをした (got injured) and 仕事を休んだ (took time off work). We need to work out who the subject of each verb is through は and が.
B with が: As my son got injured, I took time off work.
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