Singular and Plural at the same time?

[Falagar]

Hello,
I'm having trouble constructing this type of sentence:

"The one thing I forgot to buy is/are apples."

When I start the sentence with "one thing" (can also be "only thing") my instincts tell me to use "is", but because this "thing" then appears in plural form I feel the need to use "are" before it.
I feel like I've locked myself in a paradox and losing my mind.
Please Help.

[Joe]

Hello,
I'm having trouble constructing this type of sentence:

"The one thing I forgot to buy is/are apples."

When I start the sentence with "one thing" (can also be "only thing") my instincts tell me to use "is", but because this "thing" then appears in plural form I feel the need to use "are" before it.
I feel like I've locked myself in a paradox and losing my mind.
Please Help.

The verb agrees with the subject, so...

The one thing I forgot to buy is apples.

or, more idiomatically...

The one thing I forgot to buy was apples.

[Falagar]

Thank you for the reply
I considered this direction, but for some reason my brain kept telling me that "apples" can also be viewed as the subject since in this context "apples" and "thing" are one and the same. Now that you drew my attention to it I see where I was wrong.
Incidentally if I wanted to switch it around to something like:

"Apples are the one thing I forgot to buy"

"Are" would be the correct form, because I changed the subject to be "apples".

I considered using past tense, but I feel like it would put the whole situation too far in the past. In my mind I was talking about something that just happened. Does it make any sense or am I trying to invent a type of tense that doesn't exist?

[OccamsRazor]

Sometimes a plural noun is used to represent a singular entity.

For example, [A thousand dollars] is a lot of money. (single quantity)

There are a thousand dollar bills on the floor. (1,000 individual pieces)