zhè shì lājīdài.

Mandarin often follows the same basic pattern as English here:

• 这 = this
• 是 = is
• 垃圾袋 = trash bag / garbage bag

So the structure is simply:

this + is + trash bag

That makes 这是垃圾袋 a straightforward identification sentence: you are pointing something out and saying what it is.

是 is the verb to be when you are identifying something as a noun.

So in 这是垃圾袋, 是 links 这 and 垃圾袋, just like is in English:

• 这 是 垃圾袋
• This is a trash bag

A useful point: 是 is commonly used when one thing is being identified as another noun.

Examples:

• 这是老师。 = This is a teacher.
• 这是我的包。 = This is my bag.

This is a very common learner question.

By itself, 这 means this.
But when 这 comes directly before a noun, it usually needs a measure word:

• 这个垃圾袋 = this trash bag

So many learners expect:

• 这个是垃圾袋
• or 这是一个垃圾袋

However, in 这是垃圾袋, 这 is not directly modifying the noun 垃圾袋. Instead, 这 stands alone as this, and then 是 connects it to the noun:

• 这 = this
• 是 = is
• 垃圾袋 = trash bag

So 这 here is perfectly natural.

Yes, absolutely.

Both are grammatical, but they feel slightly different:

• 这是垃圾袋。 = This is a trash bag. / This is the trash bag.
• 这是一个垃圾袋。 = This is a trash bag.
  
  • 一个 makes the noun explicitly countable, like saying one trash bag or a trash bag.

In everyday speech, 这是垃圾袋 is often fine when you are simply identifying the object.
If you want to be a bit more explicit, 这是一个垃圾袋 also works.

Mandarin does not use articles the same way English does.

English often requires:

• a trash bag
• the trash bag

Chinese usually does not need a separate word for a or the.

So:

• 垃圾袋 can mean trash bag, a trash bag, or the trash bag

The exact meaning depends on context.

If you want to make a/one more explicit, you can add 一个:

• 这是一个垃圾袋。 = This is a trash bag.

垃圾袋 is a compound noun:

• 垃圾 = garbage / trash
• 袋 = bag

So literally it is garbage bag, which naturally corresponds to trash bag or garbage bag in English.

This kind of noun-building is very common in Mandarin: one noun modifies another.

Examples:

• 书包 = book + bag = schoolbag / backpack
• 茶杯 = tea + cup = teacup
• 手机袋 = phone + bag/pouch

The pinyin is:

• 这 = zhè — 4th tone
• 是 = shì — 4th tone
• 垃圾袋 = lājīdài

For 垃圾袋:

• lā = 1st tone
• jī = 1st tone
• dài = 4th tone

A few pronunciation tips:

• zh in zhè is not the same as English j. It is a retroflex sound, with the tongue curled back slightly.
• sh in shì is also retroflex, not exactly like English sh.
• j in jī is pronounced with the tongue closer to the front of the mouth than English j.
• The 4th tone in zhè, shì, and dài falls sharply, like a firm downward pitch.

So the rhythm is roughly:

ZHÈ SHÌ LĀ-JĪ-DÀI

No, not in this structure.

Measure words are usually needed when a demonstrative like 这 directly modifies a noun:

• 这个垃圾袋 = this trash bag

Here, though, the sentence is:

• 这 = this
• 是 = is
• 垃圾袋 = trash bag

Since 这 is standing alone as the subject, there is no need for a measure word there.

If you wanted to put this directly before trash bag, then you would normally use a measure word:

• 这个垃圾袋

So compare:

• 这是垃圾袋。 = This is a trash bag.
• 这个垃圾袋…… = This trash bag...

A simple way is to add 吗 at the end:

• 这是垃圾袋吗？ = Is this a trash bag?

Another common spoken pattern is 是不是:

• 这是不是垃圾袋？ = Is this a trash bag or not?

Both are natural.
For beginners, adding 吗 is the easiest method.

You can say:

• 这不是垃圾袋。

Here, 不是 is the negative form of 是:

• 是 = is
• 不是 = is not

So:

• 这是垃圾袋。 = This is a trash bag.
• 这不是垃圾袋。 = This is not a trash bag.

You would use it when identifying an object, especially while pointing to it or showing it to someone.

For example:

• explaining household items
• teaching vocabulary
• sorting rubbish
• telling someone which bag to use

It can sound like:

• This is a trash bag
• This is the trash bag
• This one is for garbage

In real conversation, context decides the most natural English translation, even though the Chinese sentence stays the same.