What is the meaning of "litter"?

Why "running" is used?

Good question! In this case, 'running' is a verb is used to describe water or liquid rapid movements. Ex: The flood water was running down the street. Ex: I love the feeling of Cola running down my throat. Ex: The water was running down him.

What does "go out" mean?

You can understand "go out" as being short for "go out on a date," which is related to the concept of dating. Thus, to "go out with someone" is commonly understood to mean "date someone" or "go out on a date with someone." Ex: I went out on a date last weekend, but it wasn't very fun. Ex: I went out with Peter during high school.

Can I use “make sure” instead of “make certain”? Any more synonyms?

Yes! You can definitely use "make sure" instead of "make certain." They both have the same meaning. Another synonym that would work in this particular sentence would be ensure. Ex: Make sure you sit towards the middle of the group. Ex: Ensure that you sit towards the middle of the group.

What does "in case" mean here?

"In case" means a precaution taken against something bad happening, such as an unforeseeable event. Your question is also correct. Saying "If someone's trying to steal..." is more formal than saying "in case someone is trying to steal..." If you are concerned which one to use, either one is perfectly fine to use as English doesn't really focus on the formality. Here are examples of "in case" being used. Ex: I like to have extra clothes just in case. Ex: She keeps a shovel in her car in case she gets stuck in the winter. Ex: We always bring food on road trips in case we can't find a place to eat.

Why is there no article before "dinner"? I thought it needs article like "have A dinner."

There isn't an article before dinner because we do not use articles before meals. We only use articles before meals when there is an adjective placed before it. We use the article "the" before a meal when referring to a specific meal. There isn't an article used here because she is not talking about a particular dinner and she doesn't use an adjective before dinner. Ex: Dinner will be ready in an hour. Ex: Are you ready for lunch? Ex: What would you like for breakfast? Ex: I ate a late lunch. Ex: We had a light breakfast. Ex: She had an early dinner. Ex: The breakfast on the cruise was spectacular! Ex: The lunch we had at the restaurant was disgusting. Ex: The dinner at Hell's Kitchen was amazing.