Ther tried to find out what kind of diseases killed the oldman There is nothing wrong with this sent

No, I don't. That's right.

They are the subject.

try is a predicate verb.

to find out...The infinitive is the object.

what kind of diseases killed the old man The whole part is again used as the object of find out.

what kind of diseases killed the old man

In this part, killed the old man is used as the postposition object of disease.

They tried to find what kind of diseases killed the oldman

After all, they still haven't found it, they have to try to find it, and find out has already been found, and out should be removed.

kind of of; A little; There are a few points.

i'm feeling kind of tired.

I feel a little tired.

what kind of; Which type.

yes, what kind of remittance?

Yes, what type of money transfer?

a kind of a; Somewhat; Sort of.

he had a kind of feeling that she would phone him.

He had a hunch that she would hit him.

he felt a kind of compunction.

kinds of

Various; Many kinds.

eighteen kinds of weapons.

He felt a little guilty.

What kind of + noun as subject: what kind of bood sells best in this bookstore?

a kind of ,this kind of + noun this kind of cloth feels soft.

tests of this kind are specially for self-taught students.

These kinds of + plural nouns are multiple of the same kind of thing, which is equivalent to the plural noun + of this kind

these kind of shoes are specially for jogging.

Kinds of + noun (plural) Expresses the multiple categories of a thing.

some new kinds of shoes are now on sale.

Foods are applied here because food is used to denote the type of food, and when it denotes multiple food types, the plural should be used.

The food table type can be used in plural.

So it's the second.

Of course, food alone is an uncountable noun.

When the food type is listed, you can count it, and choose foods

You're right: what kind of and what kind of is different from what kind of is that one is singular and the other is plural.

The nouns followed by of of kind of kinds are not related, and it is okay to take the number of orders or plural nouns according to the actual situation.

One Fuji apple and two pounds of Fuji apples are one variety

What do you want to do after graduation? Why? What kind of job are you looking for?

What do you plan to do after graduation? Why? What kind of job you can find in the future.

What do you plan to do after graduation? Why? What kind of job will you be able to find in the future?