When you can drop the subject from a sentence in informal English.
In informal English (spoken and written), it is sometimes possible to drop the subject from your sentence. Generally speaking, you can only do this if both of these two things are true:
Compare these two sentences:
Understanding the parts of speech is very important for studying a language. It helps you understand and talk about grammar and language. It also helps for putting language together. To be a good learner, make sure you know the parts of speech.