The basic answer is ... whenever you want to.
The more techical answer is whenever the topic changes. Paragraphs are like little ideas - start a new paragraph when you start a new idea. Notice how I have started a new paragraph here because I changed ideas from "whenever you want to" to "whenever the topic changes."
A paragraph can be as many sentences as it needs to be - however, you usually do not want to make one sentence a paragraph unless it is a line of dialogue. Paragraphs are usually several sentences long because you need the sentences to explain your idea or to make your point.
Here are some examples of when you need a new paragraph:
These are a few standard purposes to make a new paragraph:
When you start in on a new topic
When you skip to a new time
When you skip to a new place
When a new person begins to speak
When you want to produce a dramatic effect
When you begin a new idea or point
To contrast information or ideas
When your readers need a pause
When you are ending your introduction
When you are starting your conclusion
Yes, and you have to indent too!
Whenever you start a new topic, you must start a new paragraph. Each paragraph should be about one topic.
You should start a new paragraph with each new speaker.
Yes indent each new paragraph.
Usually not--each new speaker should start a new paragraph.
You should start a new paragraph with each new topic. Paragraphs should not be longer than a few hundred words.
No, each paragraph should be about the same subject. If you start a new subject, you need to start a new paragraph also.
You do not have to underline it. A new paragraph should start on a new line and the first line may be indented, but this is optional.
each new line of dialouge
In proofreading, the symbol used to show where a new paragraph should begin looks like: ¶.
Yes because it is a new paragraph.
Sort of... when a new person begins speaking, you should start a new line, but not a new paragraph. Hope that helped :D