What should be added when an adverb begins a sentence?

When an adverb begins a sentence, you need to add a comma after it. This is because the adverb is acting as an introductory element, and a comma helps to separate it from the rest of the sentence.

Here are a few examples:

* Carefully, she opened the box.

* Suddenly, the lights went out.

* However, I still believe in her.

Exceptions:

There are a few exceptions to this rule. If the adverb is very short (like "now" or "then") and the sentence is simple, you might not need a comma. However, it is always safer to add a comma to be grammatically correct.