이건
이건 means “this” as the topic in 이건 아주 좋아요 and 이건 너무 커요. It is used to identify the thing being evaluated or described. A useful pattern is 이건 + a description when pointing out and commenting on something.
아주
아주 means “very” in 아주 좋아요, 아주 쉬워요, and 아주 어려워요. It increases the degree of the following description. Use 아주 before an adjective when giving a strong evaluation, such as when saying that something is very good or very difficult.
좋아요
좋아요 means “is good” in 이건 아주 좋아요. It is the polite form used here to give a positive evaluation; its citation form is 좋다. You can use 좋아요 after a topic when politely saying that something is good.
그건
그건 means “that” as the topic in 그건 좋지 않아요, 그건 아주 쉬워요, and 그건 아주 어려워요. It identifies the thing being evaluated or described, often something already understood in the conversation. A useful pattern is 그건 + a description when commenting on that thing.
좋지
좋지 carries the “good” meaning inside the exact expression 좋지 않아요. The full expression 좋지 않아요 means “is not good,” so 좋지 should be understood together with 않아요 here. A useful pattern is an adjective followed by 지 않아요 for a polite negative evaluation.
않아요
않아요 supplies the negation in 좋지 않아요, making the whole expression mean “is not good.” It follows 좋지 in this dialogue to turn the positive evaluation into a negative one. Use the pattern adjective + 지 않아요 when politely saying that something is not a certain way.
너무
너무 means “too” in 너무 커요 and 너무 작아요, showing that the size is excessive for the situation. It comes before the description whose degree is excessive. Use 너무 + an adjective when saying that something is too big, too small, or otherwise beyond what is suitable.
커요
커요 means “is big” in 이건 너무 커요. It is the polite form of 크다 used to describe the size of something. You can use 커요 after a topic when politely saying that an object is big.
작아요
작아요 means “is small” in 이건 너무 작아요. It is the polite form of 작다 used to describe the size of something. You can use 작아요 after a topic when politely saying that an object is small.
쉬워요
쉬워요 means “is easy” in 그건 아주 쉬워요. It is the polite form of 쉽다 used to evaluate the difficulty of something. Use 쉬워요 when telling someone that a task, question, or activity is easy.
어려워요
어려워요 means “is difficult” in 그건 아주 어려워요. It is the polite form of 어렵다 used to describe something as difficult. Use 어려워요 when telling someone that a task, question, or activity is difficult.