yīshēng shuō chī tài duō táng shì huài xíguàn.

In Mandarin, 说 can be followed directly by the content of what is said.

So:

• 医生说吃太多糖是坏习惯。

is natural without any extra word for that.

English usually says The doctor says that..., but Mandarin often just puts the statement right after 说. A word like 说 does not need a connector here.

A simple way to break it down is:

• 医生 = the doctor
• 说 = says / said
• 吃太多糖 = eating too much sugar
• 是 = is
• 坏习惯 = a bad habit

So the overall structure is:

• 医生
  • 说
    • whole statement

And inside the statement:

• 吃太多糖
  • 是
    • 坏习惯

That means 吃太多糖 is acting like the subject of the clause 是坏习惯.

Yes, 吃 is a verb, but in Mandarin, a verb phrase can often act like a noun phrase.

So:

• 吃太多糖 literally contains the verb eat
• but as a whole, it means something like eating too much sugar

English does this too:

• Eating too much sugar is a bad habit.

So Mandarin is working in a very similar way here.

Because 太多糖 means too much sugar.

Here:

• 太 = too / excessively
• 多 = much / many
• 太多 = too much / too many

Since 糖 is the thing being quantified, 太多 comes before it:

• 吃太多糖 = eat too much sugar

You can think of it as:

• 吃 = eat
• 太多糖 = too much sugar

So the verb comes first, then the object phrase.

The difference is mainly about attitude and meaning:

• 很多 = a lot of / many / much
• 太多 = too much / too many

So:

• 吃很多糖 = eat a lot of sugar
• 吃太多糖 = eat too much sugar

太多 suggests excess, often with a negative implication. That fits this sentence because the doctor is calling it a 坏习惯.

Because 坏习惯 is a very normal adjective + noun combination.

• 坏 = bad
• 习惯 = habit
• 坏习惯 = bad habit

In Mandarin, short common adjectives often directly modify nouns without 的, especially in fixed or everyday combinations.

So 坏习惯 sounds natural.
If you said 坏的习惯, it would usually sound more marked, more specific, or less natural in this simple sentence.

是 links one thing to another, like to be in English.

Here it connects:

• 吃太多糖
  and
• 坏习惯

So the sentence is saying:

• Eating too much sugar is a bad habit.

This is a straightforward identification/judgment pattern:

• A 是 B

where A is being described or classified as B.

Yes, that is also natural, but it means something slightly different in tone.

• 吃太多糖不好。 = Eating too much sugar is not good.
• 吃太多糖是坏习惯。 = Eating too much sugar is a bad habit.

The second one is a stronger, more categorical judgment because it labels the behavior as a bad habit, not just not good.

So both are correct, but they are not exactly the same in nuance.

It can mean either, depending on context.

Mandarin verbs do not change form for tense the way English verbs do. So 说 by itself does not automatically tell you whether it is present or past.

Context decides:

• time words
• earlier sentences
• situation

So 医生说 could be:

• the doctor says
• the doctor said

If you want to make past time clearer, Mandarin often adds other clues, such as a time word or sentence context.

Mandarin does not have articles like a, an, and the in the same way English does.

So:

• 是坏习惯

can naturally mean:

• is a bad habit

The idea of a is understood from the structure and meaning, even though there is no separate word for it.

Because the sentence is making a general statement.

• 吃太多糖是坏习惯 means that, in general, eating too much sugar is a bad habit.

Mandarin often leaves out the person when the statement is generic or universally true. English can do this too:

• Eating too much sugar is a bad habit.

It does not have to say your, my, or people’s unless that detail matters.

In this sentence, 糖 means sugar.

That can refer to sugar as a substance in a general sense. Depending on context, 糖 can sometimes relate to sweets, but here the meaning is clearly sugar because of the health-related statement.

If you specifically wanted to say candy, you would often use another word such as 糖果.

So in this sentence:

• 糖 = sugar

A helpful chunking is:

• 医生说 / 吃太多糖 / 是坏习惯。

Pinyin with tones:

• yīshēng shuō chī tài duō táng shì huài xíguàn

A few pronunciation notes:

• 医生 yīshēng: first tone + neutral-sounding second syllable in connected speech for many speakers, though it is written yīshēng
• 说 shuō: first tone
• 吃 chī: first tone
• 太 tài: fourth tone
• 多 duō: first tone
• 糖 táng: second tone
• 是 shì: fourth tone
• 坏 huài: fourth tone
• 习惯 xíguàn: second tone + fourth tone

Try not to pause after every word. It sounds more natural in meaningful chunks rather than word-by-word.