An elliptical clause is a clause that is not grammatically complete. Whilst we understand the clause, a part of it is missing. It is clear to the listener what the missing words are because of the logic of the sentence. An elliptical clause can be any type of clause.
In written and spoken English, elliptical phrases/clauses are used all the time. They are considered grammatically correct and natural. Indeed it often sounds unnatural to use the full construction.
1. Omitting relative pronouns.
I knew (that) I had forgotten something.
That is the man (who) I had lunch with last week.
I suggest (that) you try Oxford Street.
We stayed at the hotel (which) John recommended.
IMPORTANT: see the rule in the relative clause definition for when these pronouns can be omitted.
2. Omitting a verb or verbal phrase in comparative phrases.
He is taller than she (is).
Susan can dance better than John (can dance).
She was as busy as I (was).
He worried about the quality more than (he worried about) the price.
IMPORTANT: See the rule in comparatives for using the object or the subject pronoun.
3. Omitting pronouns in passive phrases.
The equipment (which is) needed to tighten the bolt is attached to the frame.
4. Omitting words or phrases mentioned immediately prior to the sentence.
a. In conversations.
A. When will you print the report?
B. (I will print the report) as soon as I have finished.
A. Fred asked me to go to the management meeting this afternoon.
B. He asked me (to go to the meeting) too but I don't have the time.
b. In the same or the previous sentence.
I have three more rules to write and Tania (has) four.
I knew she had a problem but I didn't know what (the problem was).
Peter wants to go on holiday but he doesn't know where (he wants to go).
Peter is going to the meeting. Mary is going (to the meeting) too.
I would like to watch a film but I don't know which (film).
James doesn't like his boss but he doesn't know why (he doesn't like his boss).
5. Omitting verbal phrases.
Once (they were) inside the house, they felt warmer.
What (will happen) if I press this button?
It is as easy to make a cake as (it is) to buy one.
Shoot when (you are) ready.
6. Omitting auxiliary verbs.
I have visited Thailand and (I have visited) all the temples.
Mary has done all the washing and (she has done all) the ironing.
7. Omitting words or phrases in casual or colloquial speech.
In casual or colloquial speech, we often leave out many parts of speech. Different people leave out different parts of speech depending on region, upbringing and the relationship with the other person.
Consider the following conversation:
A. Do you want to go for a drink Jim?
B. Yes I do. How about going to 'The Dog and Duck' at around six o'clock this evening.
This is what you might hear:
A. Fancy a pint, Jim?
B. Yeah, Dog and Duck, say 6ish?
Further examples:
Jim: (Are you) in a hurry Mike?
Mike: Nods. (Yes I am I) don't want the boss to see me. (I have) terrible results this month.
Jim: (Have you) had lunch?
Mike: Hmmm (Yes I have) just (had lunch).
Jim: (Would you like a cup of) coffee?
Mike: Hmm (Yes please.) (I would like my coffee) black without (sugar,) please.
If there is a danger of misunderstanding, then do not leave words out of the sentence.