Relative clauses: How do that-clauses differ from what-clauses?

Question

Sometimes I can't see which clause fits the best. What "check-up" could be done to make sure which one is the right one?

For example,

1) He will do anything that is needed. or He will do anything what is needed.

2) They always ignore that is so obvious. or They always ignore what is so obvious.

3) All that I have is yours. or All what I have is yours.

My take is the correct ones are 1) that 2) that 3) what/that???

But I am not sure. I think it has to do something with different types or dependent clauses, but I don't know how to go about them in this case. Any suggestions?

Here is an interesting example

How do I know what I think, until I see what I say? VERSUS How do I know that I think, until I see what I say?

I think both are correct meaning two different things.

Answer 1

That and Which

Relative clauses with which or that usually modify nouns. The end result is that the original noun becomes one big Noun Phrase and functions just like a large noun.

• Do you remember the volcano [which erupted whilst we were on holiday]?

Here we see the noun volcano being modified by which ... holiday. The whole Noun Phrase the volcano which erupted whilst we were on holiday has the function of Direct Object of the verb remember.

Notice that it is the word which that is the subject of which erupted whilst we were on holiday - not the word volcano. Rather, the word volcano is the antecedent for the pronoun which. This means that we interpret which within the clause through the word volcano which comes before it. In normal relative clauses, the pronoun which always has an antecedent. The relative word that, also appears in clauses in front of which there is some kind of noun as an antecedent:

• Do you remember the volcano [that erupted whilst we were on holiday]?

So in both of these types of relatives clause we have an antecedent noun, which helps us interpret another element in the embedded relative clause.

What

If we compare the following sentences we''ll see that there's a very specific difference in relative clauses with what:

• The things that she did were thoughtful and useful.
• The things which she did were thoughtful and useful.
• What she did was thoughtful and useful.

or

• I liked the thing(s) which she did.
• I liked the thing(s) that she did.
• I liked what she did.

The big difference is that, as you can see, there's no antecedent noun for the relative clauses with what. The relative clauses with which/that all have the things as an antecedent. What takes the place of both the antecedent as well as the relative word.

The Examples

With regard to the Original Poster's examples:

• 1) He will do anything _ is needed.

Here we see that there is an antecedent noun phrase, anything. Therefore we need to use that or which:

• He'll do anything that's needed.

[However, if we removed the noun phrase anything completely, then we could use what:

• He'll do what's needed.]

• 2) They always ignore _ is so obvious.

Here there is no antecedent noun so we have to use what:

• They always ignore what's so obvious.

• 3) All _ I have is yours.

Here we find an antecedent noun, the pronoun all. We cannot therefore use what, but must use that instead. We could use which but when the antecedent is a pronoun like this it is often better to use that.

• All that I have is yours.

Regarding the Original Poster's last unnumbered example, consider the following:

I know what I think.

I know that I think.

The first sentence has a relative clause, what I think as a direct object. Within the relative clause itself, the object of think is understood as the item what.

The second sentence however does not have a relative clause. We know this is the case because there is no antecedent for that. The clause following know is a declarative content clause. The sentence has the same structure as I know that Yetis exist or I know that you like cheese.

In the second example,the clause that I think is the complement of the verb know. The verb think here has no object. The word that is a marker showing that the following clause is subordinate. In fact we can remove that entirely from the sentence and it will still be grammatical and still have the same meaning:

• I know I think. Or at least I believe I do.

Conclusion

To sum up, if there is an antecedent noun in the main clause then that or which appears at the front of the relative clause. We find what when there is no antecedent.

Hope this is helpful!

Answer 2

That always needs to refer to another word or clause in the context, what doesn't have to do that.

What is also used to form questions: what do you think? You don't ask "that do you think?"

A simple "trick" I use myself is that you can usually replace "what" by "that which" without changing the meaning:

1) He will do anything that is needed. or He will do anything what is needed.

That refers to anything. "He will do anything that which is needed" makes no sense. So, that is correct.

Note that without anything, what would be correct:
He will do what is needed; he will do that which is needed.

2) They always ignore that is so obvious. or They always ignore what is so obvious.

That does not refer to anything in the sentence. They ignore that which is obvious, so what is correct.

3) All that I have is yours. or All what I have is yours.

That refers to all. All that which makes no sense, so that is correct. Again, without all, it would be What I have is yours.

You last sentence is interesting:

How do I know that I think?

That refers to I think, the action of thinking. So you wonder how you can know that you actually perform the act of thinking. One could say that question is answered by the fact you ask the question!

How do I know what I think?

What can be replaced by "that which", you don't wonder if you think, but (about) what you think. Normally this version is the one you mean.

Note: as Janus Bahs Jacquet mentions, to make things a little less clear, in some dialects, what can be used instead of that in sentences like your first one. In that case, the important thing is , be consistent.