We use the singular form of the verb when the subject is:

(a) singular (b) uncountable (c) a clause

• Sarah is twenty-one today.
• The antique furniture in Mrs McDonald's old house was sold for ?4,000.
• In good dental care, what is important is regular brushing and a good mouthwash.
• What we are interested in is wheat varieties.

However, if the idea of plurality is strongly present as it is in your sentence about wheat varieties, Tanya, then this rule is not always followed. So it is quite conceivable that you might also say:

• What we are interested in are wheat varieties.

Of course, if you turn the sentence round then you have a plural subject which forces the plural form of the verb:

• Wheat varieties are what we are interested in.

The idea of plurality is also very strong in the following pair of sentences so either singular or plural verb form is possible:

• The audience was applauding like mad and stamping their feet in excitement.
• The audience were applauding like mad and stamping their feet in excitement.

there is / there are

In your example of there is, Tanya, it is as if the items are being counted separately:

There's a chair and there's a table in the room SO there's a chair and table in the room.

But note:

There are three chairs and a table in the room.

There's a table and three chairs in the room.

The general rule is that the verb form matches the item(s) that it is adjacent to:

• Either the accused or the witnesses were lying. They couldn't both be telling the truth.
• Either the witnesses or the accused was lying. They couldn't both be telling the truth.

Note that we do not usually begin sentences in English with an indefinite noun phrase. We could say:

• A knife and (a) fork were on the table.

But we usually don't. If we want to say that something exists, we usually start the sentence with the empty?grammatical word there and say:

• There's a knife and (a) fork on the table

subject-verb agreement with ‘quantity?determiners

The quantity determiners any, either, neither and none are sometimes used with a singular verb and sometimes used with a plural verb when they function as the subject of the clause.

Although the singular verb may be formally correct, in usage there is no strong preference for one or the other. So I think you may feel free to choose whichever you think best, depending, perhaps, on how strong you think the idea of plurality is.

Compare the following pairs and see which you prefer:

• I invited four policemen to my wedding but none was present.
• I invited four policemen to my wedding but none were present.
• I have two sisters but neither of them is married.
• I have two sisters but neither of them are married.
• I don't think either of them deserves a husband.
• I don't think either of them deserve a husband.
• I don't think any of the children is invited to the wedding reception.
• I don't think any of the children are invited to the wedding reception.