1. what is loose sentence and periodic sentences?
2. When do writers or speakers use loose and periodic sentences?
A "periodic" sentence is not grammatically complete until the close.
A "loose" sentence continues after grammatical completeness has been reached.
This is an example of a loose sentence:
1. The Roman moon goddess was Diana, who was the equivalent of the Greek goddess Artemis.
As you can see, the sentence is grammatically complete after "Diana":
2. The Roman moon goddess was Diana.
You can make it periodic thus:
3. Diana, the Roman moon goddess, was the equivalent of the Greek goddess Artemis.
The "periodic" style keeps the meaning in suspense until the close, and is therefore traditionally supposed to retain the attention of the listener/reader better than the loose sentence.
All the best,