Despite what your English teacher may tell you, yes. But it's not always a good idea. But of course.
we all started at the same time
I started to work out two weeks ago.
nothing
The sentence is correct if you and the article 'a': "She started her journey a long time ago."
"has started" is the simple predicate.
Yes, it is.
A simple predicate consists of only a verb or verb phrase. In this sentence the verb phrase is -- has started
"Has started" is the correct verb to use in this context. The sentence should be written as "The weekend has started."
She jumped the gun. She started at 3, when the other started at GO!
No, but you can make a sentence with the word suddenly. Suddenly it started to rain.
His name started from the letter N. This is a sentence starting from the word His.
Example sentence - I heard the thunder before the rain started.