Does it always agree with a singular verb?
For example, does one say (i) or rather (ii)?
- (i) It is the birds he is chasing away.
- (ii) It are the birds he is chasing away.
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A verb and its subject generally agree in number:
It is the birds he is chasing away.
The Great Lakes are the largest surface freshwater system on the Earth. (example from epa.gov)
In some dialects and in limited situations, agreement can be notional instead. In that case, the way you're thinking about it (as either singular or plural) is more important than its grammatical form:
%The Clash are a well-known band. (example from Wikipedia)
But agreement with it is never notional. It is always singular, so the following sentence is unacceptable:
*It are a well-known band.
Some subjects are conjoined (consisting of at least two conjuncts, A and B), in which case the verb typically† takes plural agreement, even if the individual conjuncts are singular:
He and I are a couple.
Both he and it are far too valuable to be anywhere but a secure installation. (example from COCA)
As you can see, it is sometimes followed by are in these cases because agreement is with the entire subject he and it, not just with it, and this subject is plural.
Other subjects are disjoined (consisting of at least two disjuncts, A or B), and in this case the verb generally agrees with each disjunct individually:
‹ꜱɪɴɢᴜʟᴀʀ› or ‹ꜱɪɴɢᴜʟᴀʀ› is
‹ᴘʟᴜʀᴀʟ› or ‹ᴘʟᴜʀᴀʟ› are
But sometimes singular and plural noun phrases are coordinated, and we end up with a number conflict. This conflict has to be resolved somehow, and different speakers have different preferences for doing so. For example, some prefer agreement with the plural disjunct, while others prefer agreement with the closer disjunct:
‹ᴘʟᴜʀᴀʟ› or ‹ꜱɪɴɢᴜʟᴀʀ› is ← agreement with the closer disjunct
‹ᴘʟᴜʀᴀʟ› or ‹ꜱɪɴɢᴜʟᴀʀ› are ← agreement with the plural disjunct
But if we put the plural disjunct second, then the plural disjunct is the closer disjunct:
‹ꜱɪɴɢᴜʟᴀʀ› or ‹ᴘʟᴜʀᴀʟ› are ← agreement with the closer disjunct
‹ꜱɪɴɢᴜʟᴀʀ› or ‹ᴘʟᴜʀᴀʟ› are ← agreement with the plural disjunct
So in these cases we can make both groups happy by putting the plural disjunct second. Many style guides recommend doing exactly this. However, some speakers would prefer to avoid this conflict altogether; these speakers would prefer rewriting the sentence so that they don't have to pick one or the other.
In this answer, I've used the following symbols:
% marks a sentence that is considered standard English by some speakers but not others
* marks a sentence that is considered nonstandard by all speakers
† The matter of agreement with conjoined subjects is actually more complicated than this. See e.g. Heidi Lorimor's thesis, Conjunctions and Grammatical Agreement.