6

2 played ('a' is a vowel before 'y' so the 'y' doesn't change)

3 planned ('a' is a short vowel, so we write double 'n')

4 travelled ('l' at the end of verbs is always doubled)

5 opened (no need to double 'n' because the first syllable is stressed)

6 happened (the ending is -ed)

7 studied ('d' is a consonant before 'y', so the 'y' changes into 'i')

7

2 bought

3 chose

4 fell off

5 wore

6 wrote