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. 

Sentence 1 includes the subject pronoun "I". Sentence 2 cuts, or "drops", the subject pronoun. We show the missing subject here like this:   . Sentences like 1 are considered correct standard English. That is to say, sentence 1 is a complete sentence, and is used in formal settings, like in business or school. Sentences like 2 are commonly used in informal situations, like when talking with friends and family.

You can also see people dropping subject pronouns when they write too, in text messages, Facebook status updates, tweets and other informal situations. Here are some examples:

1. Got a new car!
2. Can't make it tonight.
3. Gonna be late. 
4. Should've left earlier.
5. Gotta go!

In standard English, these sentences would be as follows:

1. I got a new car!
2. I can't make it tonight.
3. I'm going to be late.
4. I should have left earlier.
5. I've got to go.

Dropping the subject pronoun in English is said to be "bad grammar". In fact, it is very natural, and native speakers do it all the time. They just don't notice it.

When you can drop the subject pronoun

Subject pronouns are dropped in spoken English for a couple of reasons:

1. When spoken, the subject is unstressed, and that make it easy for it to disappear;
2. Context makes it clear what the missing subject is, so the meaning is still clear.

Let's look at some examples of when you can drop the subject. Remember that the location of the missing subject is shown like this:   .

1. When the subject is "I" or "we":  Sometimes you can drop the subject pronoun if you are talking about yourself, or you and some other people. Look at these examples:
   i)     Got the promotion!
   
   ii) A: Why didn't you and your family go away for the Christmas holidays?
   B:     Didn't want to.
   In the case of (i), we have dropped "I". You can do this if it is clear that you talking about yourself and nobody else. In the case of (ii), B can drop "we". It is a direct answer to a question about "you and your family". The "we" is clear from the situation.
2. When the subject is "you", "he", "she", "it" or "they": If the situation makes it clear, then "you" can also be dropped. This is usually in response to a question (so the subject is clear):
   i) A: Do you mind if I finish early?
   B:    Can if you want to.
   ii) A: Why didn't he finish the report?
   B:    Didn't have time.
   In both (i) and (ii), B is making a response to a direct question. The question sets the subject. Therefore, B does not have to repeat it. It is clear who B is talking about.
3. When the subject is "it": We use "it" as a subject to mean the weather and sometimes as a subject that doesn't add meaning to the sentence:
   i)    Was raining on the weekend.
   ii)    Seems like he forgot his computer.
   In sentence (i), "it" is dropped. It means "the weather". In sentence (ii), we drop "it". This "it" does not have any meaning. We only add it because formal English requires every sentence to have a subject. Because this "it" doesn't mean anything; we can drop it in informal English and not cause confusion.

Dropping other words from a sentence

You are also able to drop some other words from the start of a sentence in spoken English.

• If you are using a subject pronoun ("I", "you", "he" etc.) and an auxiliary verb ("had", "have", "am", "are" etc.) that can be contracted ('d, 've, 'm, 're etc), you can drop both when you are speaking:
  i) I'm going to China on Friday. →     Going to China on Friday.
  ii) You'd better go. →     Better go.
  iii) We've been waiting for about ten minutes. →     Been waiting for about ten minutes.
• We can also drop just the auxiliary verb when we ask yes/no questions. This is because the auxiliary is clear from the context, and we indicate a question with a rising tone of voice when we speak — that means the swapping of "You are" to "Are you...?" etc. is not required to show that it is a question. If the subject is clear, as in case (i), you can sometimes drop both the auxiliary and the subject:
  i) Are you going to eat that? →     You going to eat that? or     Going to eat that?
  ii) Have they been there before? →     They been there before? but not     Been there before?

Writing: dropping subjects

Dropping a subject from your sentence is most commonly done in diary English. Dropping the subject in this style of writing is sometimes called the "diary drop". Diary English is also what we use when we write on Facebook, Twitter, and our blogs, or in a text message. We are usually the subjects of our tweets and diaries and status updates, so your reader assumes that, if you drop the subject in diary English, you are talking about yourself. If you are talking about another person, then you should include a subject.

Here are some examples:

• I'm going to the gym. →     Going to the gym.
• I've been here since 7am. →     Been here since 7am.
• I would like to go to the beach. →     Would like to go to the beach.

In writing, it is best to avoid dropping words in any other situation.

When you CANNOT drop the subject

There are many cases where dropping the subject is not possible. It is ungrammatical. You can only cut words from your sentence if you do not create ambiguity. For example, you cannot do things like this:

• I'd eat it. →     Eat it.

This sentence is not clear. "Eat it" is a command. When dropping words makes your sentence unclear, you should not do it.

Here are some examples of when you cannot drop the subject, and why it is not a good idea or not possible:

Be, have, will, would and had

The auxiliary verbs, "be", "have", "will", "would" and "had" are another special case. These five words need a subject when they are affirmative, but are OK when used with "n't".