So, Too, Neither and Either

'So do I' and 'neither do I'

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I use 'so do I' to say that a positive sentence is also true for me, and I use 'neither do I' to say that a negative sentence is also true for me:

John: I hate mushrooms.
Me: So do I (=I also hate mushrooms).

Lucy: I don't live in London.
Me: Neither do I (=I also don't live in London. For example, maybe Lucy and I both live in Paris).

This is often used as a reply to someone else in a conversation, but both sentences can also be said by the same person, and even joined together:

Me: Elizabeth loves coffee. So do I.
Me: Harry doesn't play the piano and neither do I.

In my examples above, I use 'do' because the first sentence is in the present simple tense. The verb after 'so' or 'neither' changes depending on the tense of the verb in the first sentence. (This is very similar to tag questions).

Present simple: use 'do / does'	Lucy likes coffee. So do I. Lucy doesn't like coffee. Neither do I.
Present simple with 'be': use 'am / is / are'	John's at the office. So am I. John isn't at the office. Neither am I.
Present continuous: use 'am / is / are'	Luke's going out tonight. So am I. Luke isn't going out tonight. Neither am I.
Past Simple: use 'did'	Jill went to the cinema yesterday. So did I. Jill didn't go to the cinema yesterday. Neither did I.
Past simple with 'be': use 'was / were'	She was at the library. So was I. She wasn't at the library. Neither was I.
Present perfect: use 'have / has'	They've been to Colombia. So have I. They haven't been to Colombia. Neither have I.
Future simple: use 'will'	Edward will be at the cafe later. So will I. Edward won't be at the cafe later. Neither will I.
Modal verbs: repeat the modal verb	He would like a cup of tea. So would I. He wouldn't like a cup of tea. Neither would I. Emma can speak Russian. So can I. Emma can't speak Russian. Neither can I.

What about 'too' and 'either'?

We can also use 'I do too' and 'I don't either', which mean the same as 'so do I' and 'neither do I':

John: I hate mushrooms.
Me: I do too (=I also hate mushrooms).

Lucy: I don't live in London.
Me: I don't either (=I also don't live in London).

The verb changes in the same way as with 'so do I' and 'neither do I' (remember you need a negative verb with 'either'):

Present simple: John's at the office. I am too.
Present continuous: Luke isn't going out tonight. I'm not either.
Present perfect: They've been to Colombia. I have too.
Modal verbs: Emma can't speak Russian. I can't either.

'Me too' and 'me neither':

We can also use 'me too' and 'me neither'. 'Me too' has the same meaning as 'so + auxiliary verb + I' and 'me neither' has the same meaning as 'neither + auxiliary verb + I'. 'Me too' and 'me neither' are very informal:

John: I hate mushrooms.
Me: Me too (=I also hate mushrooms).

Lucy: I don't live in London.
Me: Me neither (=I also don't live in London).

Subjects other than 'I':

Of course, we can also use these expressions to talk about what's true for other people, not just ourselves:

John: I hate mushrooms.
Me: So does Laura / Laura does too / Laura too.

Lucy: I don't live in London.
Me: Neither does David / David doesn't either / David neither.

Some more examples:

We live in London and so do they.
Emma loves tennis. Jill and Laura do too.
My parents don't come here often. Neither does Alex.
She isn't French and neither is he.
You don't like cold weather. Neither do we.

Try an exercise about using 'so do I' and 'neither do I' here

So, Too, Neither and Either

Perfect English Grammar