I. ei‧ther¹ S1 W1 /ˈaɪðə $ ˈiːðər/ conjunction
either ... or
a) used to begin a list of two or more possibilities:
You add either one or two cloves of garlic.
She’s the kind of person you either love or hate.
b) used to say that if one thing does not happen then something else will have to:
It’s your choice! Either she leaves or I will!
£75 seems a lot to pay for a starter motor, but it’s either that or a new car!
c) an either-or situation a situation in which you cannot avoid having to make a decision or choice

[TahlilGaran] Dictionary of Contemporary English ▲

II. either² S1 W1 determiner, pronoun

1. one or the other of two things or people ⇒ any:
There’s tea or coffee – you can have either.
We can offer a comfortable home to a young person of either sex.
either of
Could either of you lend me five pounds?

2. used to show that a negative statement is true about both of two things or people ⇒ neither:
I’ve lived in New York and Chicago, but don’t like either city very much.
either of
There were two witnesses but I wouldn’t trust either of them.

3. either side/end/hand etc both sides, ends, hands etc Synonym : each:
He sat in the back of the car with a policeman on either side.
There are shops at either end of the street.

4. either way
a) used to say that something will be the same whichever of two things happens or is true:
You can get to Edinburgh by train or plane, but either way it’s very expensive.
b) more or less than a certain amount or measurement:
A few marks either way can make the difference between a pass and a fail.
c) used to say that someone or something does not firmly support or want either one of two things:
‘All right, let’s do that,’ said Camille, who didn’t care either way.

5. could go either way if a situation could go either way, both possible results are equally likely:
The latest opinion poll suggests the vote could go either way.


GRAMMAR
Either is used before a singular noun. Use a singular verb after it:
Either explanation is reasonable.
Either of is used before a plural noun or pronoun. In formal speech and writing, use a singular verb:
Has either of them called yet? In informal speech and writing, you can use a plural verb:
Have either of them called yet?

[TahlilGaran] Dictionary of Contemporary English ▲

III. either³ adverb

1. [in negatives] used to show that a negative statement is also true about another thing or person, or to add a different negative statement about something or someone ⇒ neither:
I haven’t seen the movie and my brother hasn’t either (=he also has not seen it).
‘I can’t swim.’ ‘I can’t either.’
It’s not an easy car to drive, and at $40,000 it’s not cheap either.

2. me either American English spoken used to say that a negative statement is also true about you:
‘I don’t have any money right now.’ ‘Me either.’

[TahlilGaran] Dictionary of Contemporary English ▲

either
adverb
1.
BAD: I shall either go home to Brazil or my family will come to England.
GOOD: Either I shall go home to Brazil or my family will come to England.
BAD: We either can go by bus or by car.
GOOD: We can go either by bus or by car.

Usage Note:
The position of either should be the same as the position of or (i.e. immediately before a subject, immediately before a main verb, immediately after a verb, etc): 'Either stay or go.' 'You should either stay or go.' 'You should stay either here or at home.' 'You should stay with either me or your uncle.'
Note that this rule applies mainly in formal styles. In everyday conversation, either often goes immediately before the main verb: 'We can either go by bus or by car.'

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2.
BAD: In fact, a motorway wouldn't either disturb the animals because they are used to cars.
GOOD: In fact, a motorway wouldn't disturb the animals either because they are used to cars.

Usage Note:
When either is used after not/never etc, it goes at the end of the clause: 'John isn't going to the party, and Ray doesn't want to go either.'

[TahlilGaran] Dictionary of Common Errors ▲