1. "Too" as an Adverb:
* No comma: When "too" acts as an adverb modifying a verb, adjective, or another adverb, you generally don't use a comma.
* Example: "He ran too fast."
2. "Too" as part of a Phrase:
* Comma: When "too" is part of a phrase like "also" or "as well," you typically use a comma before it.
* Example: "He ran fast, too." (This is equivalent to "He ran fast, also.")
3. "Too" in a Series:
* Comma: When "too" is used in a series of items, you typically use a comma before it.
* Example: "I like apples, oranges, and bananas, too."
4. "Too" for Emphasis:
* Comma: You can use a comma before "too" for emphasis, even if it's not strictly grammatically required. This is more common in informal writing.
* Example: "He was tired, too."
In Conclusion:
The best way to decide whether to use a comma before "too" is to consider its function in the sentence and the overall flow of the sentence. If you're unsure, it's always a good idea to err on the side of using a comma, as it's generally more acceptable than omitting one where it's needed.