wǒ bù mài zhège dōngxi.

This follows the basic Mandarin pattern:

subject + negative word + verb + object

So here:

• 我 = I
• 不 = not
• 卖 = sell
• 这个东西 = this thing / this item

So literally it is:

I + not + sell + this thing

That is normal Mandarin word order. Unlike English, you do not need a helper verb like do. Mandarin says I not sell this thing, not I do not sell this thing.

This is a very common confusion.

• 卖 mài = to sell
• 买 mǎi = to buy

They sound similar, but the tones are different:

• 卖 mài = 4th tone, falling
• 买 mǎi = 3rd tone, dipping

So:

• 我不卖这个东西 = I’m not selling this thing.
• 我不买这个东西 = I’m not buying this thing.

A good memory trick:

• 卖 = money goes outward from the seller to the market
• 买 = money comes inward to get something

Not a perfect mnemonic, but it helps some learners.

In Mandarin, 不 and 没 are both negatives, but they are used differently.

不

Usually negates:

• general facts
• habits
• intentions
• present or future actions
• refusals

没

Usually negates:

• completed actions
• possession in some cases
• have not done

So in this sentence:

我不卖这个东西
means something like:

• I do not sell this item.
• I’m not selling this item.
• I won’t sell this item.

If you said:

我没卖这个东西 that would usually mean:

• I didn’t sell this item.
• I have not sold this item.

So 不 is the correct choice if the sentence is about a general refusal, present situation, or future intention.

In isolation, 不 is 4th tone: bù.

But there is an important tone change rule:

• When 不 comes before another 4th-tone syllable, it changes to 2nd tone.

Since 卖 mài is 4th tone, 不卖 is pronounced:

bú mài

It is still written 不卖, but spoken as bú mài.

So the full sentence is pronounced roughly:

wǒ bú mài zhège dōngxi

Because Mandarin usually needs a measure word / classifier after 这 or 那 when directly modifying a noun.

The pattern is:

这 / 那 + measure word + noun

So:

• 这 + 个 + 东西
• 这个东西 = this thing / this item

Here:

• 这 = this
• 个 = a very common general classifier
• 东西 = thing / item

So 个 is not extra. It is required by normal Mandarin grammar.

You can think of 这个东西 as:

this + classifier + thing

English does not do this, but Mandarin usually does.

Yes, you often can, especially in spoken Chinese.

So both are possible:

• 这个东西
• 这东西

Difference in feeling

• 这个东西 sounds a bit more neutral and complete.
• 这东西 sounds more conversational and sometimes slightly more informal.

In many everyday situations, native speakers do omit the classifier in casual speech with some common nouns, but for learners, 这个东西 is a very safe and standard form.

So if you are unsure, use 这个东西.

Here, 东西 means:

• thing
• item
• object
• stuff

It is a very common everyday word.

So 这个东西 can mean:

• this thing
• this item
• this object

A fun fact: the characters literally mean east-west, but as a word together, 东西 commonly means thing/stuff. You should learn it as one vocabulary word, not as the separate meanings of 东 and 西.

Yes. Mandarin verbs do not change form for tense the way English verbs do. That means a sentence like this can cover several time-related meanings depending on context.

我不卖这个东西 could mean:

• I’m not selling this item.
• I don’t sell this kind of thing.
• I won’t sell this item.

The exact meaning depends on the situation, tone of voice, and surrounding context.

For example:

• In a shop conversation, it may mean I’m not selling this item.
• In a negotiation, it may mean I won’t sell this item.
• In a general statement, it may mean I do not sell this kind of thing.

Mandarin often leaves tense to context rather than verb endings.

Yes, 个 by itself is normally pronounced gè.

But in very common combinations like 这个, the pronunciation is often reduced in natural speech:

• careful/underlying form: zhè ge
• very common spoken form: zhège

Learners will often see it written as zhège, and that is perfectly normal in pinyin for this word.

So just learn:

• 这个 = zhège

Similarly:

• 那个 is often pronounced nàge

This is one of those very common spoken patterns that becomes standard.

Sometimes, yes.

Mandarin often drops the subject when it is already obvious from context.

So in a conversation, someone might simply say:

不卖这个东西。

That can mean:

• Not selling this.
• I’m not selling this item.
• We don’t sell this item.

The listener figures out the subject from context.

However, for learners, keeping 我 is helpful because it is clear and complete:

我不卖这个东西。

It is natural, but whether it is the best choice depends on context.

这个东西

Very general:

• this thing
• this item

Other possibilities

Depending on what you mean, a native speaker might also say:

• 这个 = this one
• 这个商品 = this product
• 这件东西 = this item/thing
• 这个货 = this merchandise/goods

So 这个东西 is natural if you mean a general object or item and do not need to be very specific.

In English, this thing can sometimes sound vague. In Chinese, 这个东西 is often perfectly normal.

Yes, it is grammatically fine, but by itself it can sound quite direct.

我不卖这个东西。
= I’m not selling this item.

If you want to sound softer or more polite, you could add polite wording, for example:

• 不好意思，我不卖这个东西。
  Sorry, I’m not selling this item.

• 这个东西我不卖。
  As for this item, I’m not selling it.
  This can sound slightly more deliberate or emphatic depending on tone.

• 这个东西不能卖。
  This item can’t be sold.
  This sounds more like a rule or restriction than a personal refusal.

So the original sentence is correct, but politeness in Mandarin often comes from added wording and tone, not just grammar.