ふとい/ふとる/ふとっている/ふとくなる

Today’s Question

Somebody wanted to say “Mum (has) put some weight on.”
She thought of the following 2 sentences:
A: 母ははは太ふとった and B: 母ははは太ふとくなった

Which of the following 1, 2, 3 or 4 is correct?

1. A is correct and B is wrong.
2. B is correct and A is wrong.
3. A and B are both correct.
4. A and B are both wrong.

Today’s Topic

Today’s topic is difference between 太ふとい and 太ふとる. If you understand the difference between the two, you will not be confused between 太ふとい and 太ふとっている, 太ふとる and 太ふとくなる ever again.

太ふとい is an いadjective meaning:

1. (for something slender like a stick, line, etc. to be) big, thick, bold, pudgy
2. (for voice to be) deep
3. (for attitude to be) shameless, impudent

An adjective is a word that describes an attribute of a noun, so 太ふとい is used to describe the condition of “something” being fat/big.

太ふとる is an intransitive verb meaning:

1. (for a person or an animal) to put on weight, to get fat
2. (for a person) to become rich, to acquire wealth

太ふとる is a verb describing a change from being not fat/big to being fat/big, so if you change 太ふとる into 太ふとっている, 太ふとっている should describe a condition of a person or an animal after gaining weight, i.e. “being fat/big.”

When you are describing the condition/status of being fat/big

Here what you have to remember is 太ふとい is for an object and 太ふとっている for a person or an animal.

This pencil is fat.
（✓）この鉛筆えんぴつは太ふとい。
（×）この鉛筆えんぴつは太ふとっている。

Please draw a thick line.
（✓）太ふとい線せんを引ひいてください。
（×）太ふとっている線せんを引ひいてください。

My legs are fat.
（✓）私わたしの脚あしは太ふとい。
（×）私わたしの脚あしは太ふとっている。

My father is fat.
（？）父ちちは太ふとい。
（✓）父ちちは太ふとっている。

That fat dog is mine.
（×）あの太ふとい犬いぬは私わたしの犬いぬです。
（✓）あの太ふとっている犬いぬは私わたしの犬いぬです。
（✓）あの太ふとった犬いぬは私わたしの犬いぬです。(In a contact clause, 太ふとっている and 太ふとった mean almost the same.)

The reason why I put “?” instead of “×” for 父ちちは太ふとい is that if I am simply comparing the thickness between a person’s body and a tree trunk, 太ふとい can be used. Also, if I’m thinking of Dad’s body to be an object that should be slender, it is not wrong to use 太ふとい there, either, although I think it’s better to avoid it as thinking of a person’s body as an object is not that nice.

When you are describing a change of becoming big/fat

Now if you want to describe a change, for things, we use 太くなる and for a person or an animal, we use 太る. (For explanation about [いadjective stem] + く + なる, please read this post.)

This branch has grown thicker.
（✓）この枝えだは太ふとくなった。
（×）この枝えだは太ふとった。

My legs got fatter.
（✓）私わたしの脚あしは太ふとくなった。
（？）私わたしの脚あしは太ふとった。 – this gives an impression that the speaker is considering his/her own legs as an independent organism – I understand sometimes we feel like talking like that!

My father put some weight on.
（×）父ちちは太ふとくなった
（✓）父ちちは太ふとった。

Answer to Today’s Question: 1

Because we are talking about Mum’s change from being thin to being big/fat, it has to be 太ふとった.

For more information about the kanji 太, click here.