okay I'm a little lost on this one :/

which choice correctly completes the sentence?
-Gently, they ______ the tiny eggs back in the nest.
*set
had setted
sat
had sat(this one looks like it could work too)

how do I know if the sentence is in present or past tense?

Your choice (set) is correct.

Make sure you are clear on the differences in the forms and uses of "sit" and "set."
http://www.getitwriteonline.com/archive/060602SitSet.htm

To determine whether the sentence is in present or past tense, you can look for verb forms that indicate the time of the action. In this case, the sentence is discussing an action that has already happened, so it is in the past tense.

Let's examine the verb options provided:

1. "Set" is the correct choice. It is the present tense form of the verb "set." However, since the sentence is in the past tense, we need to use the past tense form of the verb. Therefore, "set" alone is not the correct choice.

2. "Had setted" is an incorrect choice. The word "setted" is not a valid past tense form of the verb "set." The past tense form of "set" is "set."

3. "Sat" is an incorrect choice. While "sat" is the past tense form of the verb "sit," it does not fit in the context of the sentence. The sentence describes the action of gently handling and placing the tiny eggs, not sitting on or occupying them.

4. "Had sat" is an incorrect choice. While "had sat" is a past perfect tense form of the verb "sit," it is not suitable in this context. The past perfect tense is used to describe an action that happened before another past action, but the sentence does not include any indications of a preceding past action.

By analyzing the options, we can conclude that the correct choice is "set" since it is the appropriate past tense form of the verb "set" that matches the sentence's context.