Which ones are considered correct in these two sentences? " what I need is/are a car and a house." and " What I like is/are the mountains"

Do they always take a singular verb (is) even if they refer to plural nouns?

Solution 1:

The short answer is that in that exact phrasing of each sentence, only the singular verb works. However, the plural can work, depending on interpretation, if the sentences are rephrased.

One interpretation of the first example can be paraphrased:

The only thing I need is ^{a car and a house.}
My need is ^{a car and a house.}

→ ✔ What I need is a car and a house.

As for its other interpretation:

The two things I need are ^{a car} and ^{a house.}
My needs are ^{a car} and ^{a house.}

→ ✘ What I need are a car and a house.

Although the second interpretation breaks each noun into its own need, the specific syntax doesn't allow for what I need are. (Note that what she needs are would be equally incorrect; it's not the conjugation of the verb at issue here but the syntax around it.)

If you want to express two different needs, you have to actually rephrase the sentence in order to make the plurality of the noun need (rather than multiple objects of the verb need) obvious.

A similar analysis could be done for the second sentence, where you can interpret the mountains as a collective noun or as individual nouns.

However, as with the first sentence, only the following is a specifically correctly phrased sentence:

What I like is the mountains.

As before, rephrasing would be required in order to make the plurality of the noun like idiomatic.

For instance (although odd):

The things I like are (each of) those mountains.
My likes are (each of) those mountains.

The second of the two sentences might actually sound natural in the context of Facebook or Twitter, where likes is commonly used as a plural noun in this sense.

More natural wording would be something like this, which implies individual likes although it doesn't make it as explicit: