Sure, let's identify the words or phrases that need to be changed for each sentence to be correct:

1. I (A) discovered something (B) interestingly (C) in the store so I (D) got in to buy it.
- (B) "interestingly" should be changed to "interesting."
- Corrected sentence: I discovered something interesting in the store so I got in to buy it.

2. (A) Would you mind (B) help me solve these mathematics (C) problems? I can hardly understand (D) how to do them.
- (B) "help" should be changed to "helping."
- Corrected sentence: Would you mind helping me solve these mathematics problems? I can hardly understand how to do them.

3. She was (A) having dinner (B) when a man (C) was coming and giving her a (D) present.
- (C) "was coming and giving" should be changed to "came and gave."
- Corrected sentence: She was having dinner when a man came and gave her a present.

4. Some people (A) grow so (B) taller (C) that their health (D) may become endangered.
- (B) "taller" should be changed to "tall."
- Corrected sentence: Some people grow so tall that their health may become endangered.

5. After (A) said goodbye (B) to my parents, I (C) went to school (D) with my sister.
- (A) "said" should be changed to "saying."
- Corrected sentence: After saying goodbye to my parents, I went to school with my sister.

6. The (A) phone (B) rung while I (C) was washing the (D) dishes.
- (B) "rung" should be changed to "rang."
- Corrected sentence: The phone rang while I was washing the dishes.