using colons

At 9:10 this morning, we'll be leaving for the natural history museum.

Please bring the following materials with you: pencils, paper, erasers, and a notebook.

The bus will be back at 4 am to pick us up.
The bus will be back at 4:00 a.m. to pick us up.

Correct.

Colons are used in the sentences provided to introduce lists and to indicate a time. Let's break down each example to understand how to use colons correctly.

1. "At 9:10 this morning, we'll be leaving for the natural history museum."
Here, the colon is used to indicate a time. It separates the introductory clause ("At 9:10 this morning") from the rest of the sentence ("we'll be leaving for the natural history museum").

2. "Please bring the following materials with you: pencils, paper, erasers, and a notebook."
In this sentence, the colon is used to introduce a list. It comes after the phrase "the following materials with you" and indicates that a list of items will follow.

3. "The bus will be back at 4 am to pick us up."
This sentence does not require a colon because there is no need to introduce a list or indicate a time.

4. "The bus will be back at 4:00 a.m. to pick us up."
Here, a colon is used to indicate a time. It separates the hour ("4") from the minutes ("00"), showing that it is 4:00 a.m.

So, in summary, colons are used to introduce lists and indicate time. To correctly use a colon, place it after an introductory phrase or word, and before the list or time that follows.