Question

While the children _____ (play) by the lake, one of them _____(drop) his ball and ____ (try) to get it out himself.

Answer 1

were playing, dropped and tried...
hope it helps

Answer 2

While the children were playing (play) by the lake, one of them dropped (drop) his ball and tried (try) to get it out himself.

Reasons supporting above mentioned blanks:

• Were + playing resembles the past continuous tense. The situation has already occurred effects are still there.
• This sentence resembles simple past tense.
• This sentence resembles simple past tense.

About tenses:

• Past continuous tense is also known as past progressive tense.
• It is also known as preterite, used to talk about an already completed action