Pattern
Topic，comment
Core idea
Topic-comment order starts with what the sentence is about, then says something about it. The topic is often known, visible, or already active in the conversation.
This lets Mandarin organize information by shared attention, not only by grammatical subject.
What makes a good topic
After the topic is introduced, the comment may not need to repeat it with a pronoun. Listen for what the speaker places first, because that often tells you what the sentence wants you to focus on.
The topic can be a person, thing, event, time, place, or even an object that English would place later in the sentence. The comment then gives the useful information about that topic.
This pattern is not an excuse to scramble words randomly. The topic must be something the listener can identify, and the comment must make sense as a statement about it.
When the topic is a whole category or a list of more than one item, Mandarin often uses 都 in the comment to show that the statement applies to all of it.
Compare with time-first sentences
Time words often come early for a similar reason: they set the frame before the main comment. Topic-comment sentences are broader, but the reading habit is related. First ask what has been placed in front, then ask what the sentence says about it.
Examples
这本书，我喜欢。
Zhè běn shū, wǒ xǐhuān.
This book, I like it.
The book is introduced as the topic before the comment.
晚饭，我们六点吃。
Wǎnfàn, wǒmen liù diǎn chī.
Dinner, we eat at six.
这个问题，我不知道。
Zhège wèntí, wǒ bù zhīdào.
This question, I do not know.
Common mistakes
Avoid
这本书我喜欢它。
Use
这本书，我喜欢。
After the topic is established, a pronoun is often unnecessary.
Avoid
我不知道，这个问题。
Use
这个问题，我不知道。
The topic comes first when you use this structure.