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:

1. the word you drop is a pronoun; and
2. it is the first word in the sentence.

Compare these two sentences:

1. I went for a drink with a friend from work; and
2. Went for a drink with a friend from work.

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.