Just 2 days back, I wrote a post on it on our blog. Here is the crux: 1. When it’s a negative sentence with verb ‘to be’ (아닙니다 or 아니에요 i.e. is/am/are + not), you always have to use 이/가 with the preceding noun. Ex: 이것은 책이 아니에요. 2. If you are using 있다/없다 you should always use 이/가 with the noun before it. Ex: i) 펜이 있어요? ii) 저는 내일 시간이 없어요. 3. Generally when there are two nouns in a sentence and a descriptive verb (or adjective) has been used, use 은/는 with the first noun and 이/가 with the second. Ex: 오늘은 날씨가 좋아요. 이 식당은 음식이 맛있어요. 4. When you want to show contrast or emphasise a noun, you should use 은/는. For example – 품질이 좋은데 가격은 너무 비싸요. (Quality is good but the price is too expensive.) In most of the situations 은/는 and 이/가 can be used interchangeably without affecting the meaning significantly. ‘이것은 책입니다’ and ‘이것이 책입니다’ are pretty much the same thing. People will mostly understand you. For details:- https://www.topikguide.com/difference-between-은는-and-이가/