Disclaimer: I’m not a teacher, but I think I can help with your question.
The Japanese language has a much smaller set of sounds compared to English, for example. This is why you’ll frequently encounter words that sound the same, but mean completely different things, called homophones. It’s all context based. This is one reason why Kanji is so important when reading Japanese.
In the case of the phrase you’re talking about, お腹が空く(おなかがすく), you’ll notice that the Kanji for “Sky” (空), which also means “Empty”, is being used. Here the word 空く is a verb, and thus when you add the “masu” ending, it becomes SUKIMASU. Then you make it past tense and you end up with SUKIMASHITA. The literal translation of お腹が空く is “empty stomach”. But, obviously, a better translation would be “I’m hungry”.
Japanese will make more sense once you start learning Kanji in JFZ Book 3, and on. The end of Book 3 is also we’re you’ll learn how to further conjugate verbs, which is really where things start to open up in a huge way!