6

These are electric cars that is cars that run on electricity.

or

These are electric cars that are cars that run on electricity.

To clarify cars that run on electricity does not add any new information, it merely defines/clarifies what an electric car is.

Sorry if this is stupid I've actually never heard people use that are for definitions.

3

The phrase 'that is' does not change when introducing plural things. You could consider it to be short for 'that is to say'.

Carers, that is people who are providing unpaid care to a family member or friend

Example

That is (to say)

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2

The construction of "that is" in this sentence is an ellipsis used in a clarification rather than a use of a relative pronoun.

There are electric cars, that is to say, cars that run on electricity.

It clarifies what is meant by the term "electric car" and may get shortened to

There are electric cars, that is, cars that run on electricity.

It is of course possible to use a relative clause to clarify meaning. So you could say

There are electric cars that are cars that run on electricity.

Notice, however, the parenthetical "that is" is set off by commas, which reflect a slight pause in spoken English.

However, as you point out, the whole sentence is quite clunky because it is redundant.

Electric cars [obviously] run on electricity.

EDIT: Because a sentence seems to be poor English, people frequently ask whether the sentence contains grammatical errors without considering whether the sentence may be clumsily constructed. This question is an example. You wondered about the grammar because your ear for English correctly suggested that it was a terrible sentence. But not all terrible sentences are ungrammatical. It is quite possible for bad English to be flawless grammatically.

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  • should there be commas before and after that is? I used electric car just to be completely clear the latter part did not add information – Hao S Oct 25 '19 at 5:12
  • @Hao I understand that you just made up an example that really does not need a clarification. But the locution of "that is [to say]" is to signal the clarifying interruption of a parenthetical word, phrase, or clause. Punctuation is not uniform: it is determined by style guides. The usual practice, however, is for interruptions to a sentence's logical flow such as "however" to be marked off by commas (or parentheses). – Jeff Morrow Oct 25 '19 at 11:58

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