Learn Hardcore Japanese: satou no ryou ga oosugiru to, hutorimasu. - If the amount of sugar is too high, you gain weight.

Grammatically, 太ります has no explicit subject here.
In Japanese, the subject is often omitted when it is clear from context or when making a general statement.

In this sentence:

砂糖の量が多すぎると、太ります。
If the amount of sugar is too much, (you/people in general) get fat.

Possible understood subjects:

“you” (generic you, like in English: “If you eat too much sugar, you get fat.”)
“people” in general
“I” or “we”, depending on context

So the subject is implied, not stated. In a textbook-style “general truth” sentence, it’s usually understood as “people / you in general.”

Why is it 砂糖の量 and not just 砂糖? What does の do here?

の links two nouns and often means “of” or shows possession/association.

砂糖 – sugar
量 – amount
砂糖の量 – amount of sugar

If you said just:

砂糖が多すぎると、太ります。

it would mean:

If there is too much sugar, you get fat.

This is also natural and commonly used.

The nuance:

砂糖が多すぎる – “There is too much sugar” (focusing on sugar itself)
砂糖の量が多すぎる – “The amount of sugar is too much” (more explicitly focusing on the quantity)

Both are fine; 砂糖の量 just makes the idea of “amount” explicit.

Why is the particle が used after 量 instead of は or を?

In the clause:

砂糖の量が多すぎる

砂糖の量 – “the amount of sugar”
多すぎる – “is too much”

Here, 砂糖の量 is the subject of the adjective phrase “is too much.”
So が marks it as the subject:

Xが多すぎる – “X is too much”

You could sometimes see 砂糖の量は多すぎる in a different context, where you’re contrasting or theming it:

砂糖の量は多すぎる。
As for the amount of sugar, it’s too much.

But in the conditional clause 砂糖の量が多すぎると, が is the standard, neutral subject marker. を wouldn’t work here, because 量 is not a direct object of a verb; it’s the thing that is too much.

What exactly does と mean here? I thought と was mainly “and” or a quotation marker.

と has several uses. In this sentence it is a conditional:

[non-past clause] + と、[result].
When/if [X happens], [Y happens].

So:

砂糖の量が多すぎると、太ります。
→ If (or when) the amount of sugar is too much, you get fat.

Key points about と as a conditional:

It often describes automatic / natural / predictable results:
- 春になると、暖かくなります。
  When it becomes spring, it gets warm.
It sounds more like a general rule or law, not a one-time event.
You can’t normally use it for a one-time, volitional action by the speaker:
- × 暑いと、窓を開けます。 (sounds odd as a personal decision)
- ✓ 暑いと、アイスがよく売れます。 (When it’s hot, ice cream sells well.) – general tendency

In this sentence, “If there’s too much sugar, people get fat” is being presented like a general rule, so と is natural.

How does 多すぎる work? Why is it not 多いすぎる?

すぎる is a verb that means “to be too much / to over‑~” and it attaches to adjectives and verbs in a specific way.

For い‑adjectives (like 多い):

Remove the final い.
Add すぎる.

So:

多い (many / much)
→ 多すぎる (too many / too much)

That’s why 多いすぎる is wrong: the い has to be dropped.

Other examples:

高い → 高すぎる – too expensive / too tall
早い → 早すぎる – too early / too fast

For verbs, you attach すぎる to the verb stem:

食べる → 食べすぎる – to eat too much
飲む → 飲みすぎる – to drink too much

What is the nuance difference between 多い and 多すぎる here?

多い – “a lot / many / much” (a large amount, but not necessarily a problem)
多すぎる – “too much / too many” (more than desirable; excessive)

So:

砂糖の量が多いと、太ります。
If the amount of sugar is a lot, you get fat.
(Stylistically a bit odd; “a lot” but not clearly too much)
砂糖の量が多すぎると、太ります。
If the amount of sugar is *too much, you get fat.
(Clearly says the sugar is *excessive and causes a problem.)

In natural Japanese, when you want to express a health risk or negative outcome, 多すぎる sounds more appropriate.

Why is 太ります used instead of 太い? Aren’t they both about “fat”?

They’re related but different parts of speech and meaning:

太る (verb) – to gain weight / to get fat
- 太ります – polite non-past form: “get fat / will get fat”
太い (adjective) – thick / fat (describes the state of something)

So:

太ります – describes the process / change: someone gains weight.
太いです – describes the state: something is fat/thick.

In this sentence, we’re talking about the result of eating too much sugar over time: you become fatter (gain weight). So the verb 太る (→ 太ります) is the natural choice.

Using 太い here would be ungrammatical:

× 砂糖の量が多すぎると、太いです。
This doesn’t make sense (“if the amount of sugar is too much, [I] am thick”).

Why is the verb in the non-past 太ります used to mean “will get fat” or “get fat (in general)”?

Japanese non-past (dictionary form / 〜ます form) covers both:

present / habitual actions
future actions

太ります can mean:

get fat (in general / habitually)
will get fat (as a predicted future result)

In this kind of generic conditional statement:

砂糖の量が多すぎると、太ります。

the non-past is used to express a general truth / tendency:

If the amount of sugar is too much, you (people) get fat.
If you have too much sugar, you will get fat.

Japanese doesn’t need a special future tense for this; the context gives the “future/inevitable result” feeling.

Could I say 砂糖をとりすぎると、太ります。 instead? What’s the difference in nuance?

Yes, that’s a very natural sentence:

砂糖をとりすぎると、太ります。

Breakdown:

砂糖をとる – to take sugar / to consume sugar
とりすぎる – to take/consume too much
Whole: If you consume too much sugar, you get fat.

Difference in nuance:

砂糖の量が多すぎると、太ります。
Focuses on the amount of sugar being too much (more descriptive/neutral).
砂糖をとりすぎると、太ります。
Focuses on the action of consuming too much sugar (more about what you do).

Both are correct and natural. The original sentence sounds a bit more like describing a condition (“when the amount is excessive”) rather than an action (“when you eat too much”).

Is the comma (、) before 太ります necessary? Does it change the meaning?

The comma (、) in:

砂糖の量が多すぎると、太ります。

is not grammatically necessary. It’s mainly for readability and pacing.

You could also write:

砂糖の量が多すぎると太ります。

and the meaning is the same.

The comma just visually separates the conditional clause from the result clause, much like a comma in English after “If…”

How would I make it clearly about me (I will get fat) instead of people in general?

You can add 私 and maybe は to explicitly mark yourself:

砂糖の量が多すぎると、私は太ります。
If the amount of sugar is too much, I get fat.

Or:

私は砂糖の量が多すぎると、太ります。

Both are fine. The position of 私は just changes the emphasis slightly, but the meaning is essentially the same.

You could also make it feel more personal/emotional by using 太ってしまいます (“end up getting fat”):

砂糖の量が多すぎると、太ってしまいます。
If the amount of sugar is too much, I end up getting fat (unfortunately).