Japanese language has different grammatical styles from English. Sometimes it is said that Japanese is an "non-subject-language". Your question is a textbook example. It is famous as the so-called 象鼻文(ぞうはなぶん). 象は 鼻が 長い An elephant has a long trunk. (Elephants have long trunks.) Many Japanese scholars who had studied English grammar have pazzled over this sentence for many years. "Which word is the subject?" It's clear that the subject is "elephant" in English translation. But, it's 象は長い鼻を持っている in Japanese. It never can be equal to 象は鼻が長い. An famous amateur scholar had explained that there's no "subject" in such sentences. There're only "主題(topic)" and "述部(predicate)". I think it's an elegant explanation, too. In this way, 象は鼻が長い can be divided into two parts; 象は(topic) 鼻が長い(predicate). And it can be translated into English as "As for an elephant, the trunk is long". So 軽い病気やけがは店で買った薬で治すと税金が安くなる also can be explained same way. Topic =what we are talking about = 軽い病気やけが Predicate =the content of the talk =税金が安くなる …店で買った薬で治すと can be regarded as a condition of the predicate. So it can be translated into English in this way; As for minor illnesses and wounds, taxes can be cheaper if you use the medicine which you bought from shops. ...Mr. René's translation also is righteous. But 軽い病気やけが is NOT the topic of "the first sentence" , but the topic of whole sentence.

軽い病気やけが (topic) は (topic particle) 店で買った薬で治すと 税金 (subject) が (subject particle) 安くなる 軽い病気やけが is the topic of "the first sentence" which is connected with と to the second part where the subject is 税金. curing minor illnesses and wounds with medicine bought from shops will reduce tax