While I was swimming a little further down, I saw green sea turtles.

The first action is in the past continuous tense. That means, the action in the above sentence, swimming, took place over a period of time.

Saw green sea turtle is in the simple past tense and it is the single action. We can understand that Monita started swimming before she saw green sea turtles.

When we combine the two sentences using past continuous and past simple, we sometimes use the conjunctions while or when.

While I was swimming a little further down, I saw green sea turtles.

If we change the order of our example above, we get: When I saw green sea turtles, I was swimming a little further down.

We use while in a clause with continuous action, using a verb in a continuous tense. We use when in a clause with a single action, using simple past tense.

We can put while and when either at the beginning of a sentence or in the middle. While I was swimming a little further down, I saw green sea turtles.

I saw green sea turtles while I was swimming a little further down.

When I saw green sea turtles, I was swimming a little further down.

I was swimming a little further down when I saw green sea turtles.

Can you see a difference in the above sentences? Both sentences have a similar meaning. In the first sentence, we just need a comma to separate the two clauses.

b. Underline the first actions in these sentences. Number one has been done for you.

1. While I was struggling to put on the mask, the snorkel, and the fins, one of the participants approached me and offered help.

2. When we arrived at the snorkeling site, many snorkelers were getting ready to start swimming.