fēngxiāng yǐqián xiě biāoqiān, yǐmiǎn ná cuò.

In Chinese, 以前 often comes after a word or phrase to mean before that / before doing that.

So:

• 封箱以前 = before sealing the box
• literally: seal-box before

This is a very common pattern:

• 吃饭以前 = before eating
• 睡觉以前 = before sleeping
• 出门以前 = before going out

So here, 封箱以前写标签 means write the labels before sealing the box.

It is functioning as a time phrase.

• 封箱 = sealing the box / closing up the box
• 以前 = before

Together, 封箱以前 sets the time for the main action:

• 封箱以前，写标签
  = Before sealing the box, write the labels

Chinese often uses this kind of phrase at the beginning of a sentence to give the time context first.

Chinese very often omits the subject when it is obvious from context.

In a sentence like this, it sounds like an instruction, reminder, or procedure step, so the subject is understood:

• (你/你们) 封箱以前写标签，以免拿错。

English usually needs a subject, but Chinese often does not if the listener already knows who is supposed to do the action.

This makes the sentence sound natural and efficient.

Good question. These are related but not identical:

• 写标签 = write the label / write on the label
• 贴标签 = stick/attach the label onto something

So 写标签 focuses on writing the identifying information on the label.

Depending on context, a Chinese speaker might also say:

• 封箱以前先写标签
• 封箱以前先贴标签
• 封箱以前先把标签写好

If the important point is to mark the box correctly, 写标签 is perfectly natural. In real life, the full process might involve both writing and attaching the label.

以免 means something like:

• so as to avoid...
• in order not to...
• to prevent...

It is used before an unwanted result.

In this sentence:

• 以免拿错
• = so as to avoid taking the wrong one

This structure is common:

• 早点出发，以免迟到。
  Leave early, so you won’t be late.
• 先保存文件，以免丢失。
  Save the file first, to avoid losing it.

So 以免 introduces the bad thing you want to prevent.

Because the object can be omitted when it is obvious.

Here, the context is boxes and labels, so 拿错 naturally means:

• take the wrong one
• pick up the wrong box
• get the wrong package

Chinese often leaves out the object if the listener can easily infer it.

You could say:

• 以免拿错箱子
• 以免拿错包裹

But 以免拿错 is shorter and very natural when the situation is already clear.

拿错 is a common verb + result complement structure.

• 拿 = take
• 错 = wrong / by mistake

So 拿错 means:

• take the wrong one
• take something by mistake

This pattern is very common in Mandarin:

• 看错 = see wrongly / misread
• 听错 = hear wrongly / mishear
• 写错 = write wrongly / make a mistake in writing
• 买错 = buy the wrong thing

So 错 after a verb often shows that the action was done incorrectly or to the wrong target.

Because this sentence is giving a general instruction or warning, not describing a completed event.

• 以免拿错 = so as not to take the wrong one

If you said 拿错了, that would sound more like:

• ended up taking the wrong one
• already took the wrong one by mistake

In procedural or instructional language, Chinese often uses the plain verb form without 了.

Compare:

• 写清楚一点，以免别人看错。
  Write it more clearly so people don’t read it wrong.
• 他拿错了。
  He took the wrong one.

Yes. 在 can be added, and it is grammatically fine:

• 在封箱以前，写标签。

But in many cases, Chinese simply leaves 在 out, especially in short instructions or written reminders.

So both are possible:

• 封箱以前写标签
• 在封箱以前写标签

The version without 在 is a little more compact.

Yes, in many contexts you can.

• 封箱以前
• 封箱之前

Both mean before sealing the box.

That said, 之前 often sounds a little more straightforward in modern everyday speech, while 以前 can also have the broader meaning in the past depending on context.

Here, because it follows a verb phrase, 以前 clearly means before.

So both are natural here, but 封箱以前 is completely correct.

It can be understood as either singular or plural depending on context.

Chinese nouns usually do not have to show singular/plural the way English does.

So 写标签 could mean:

• write a label
• write labels

The context decides. If you are sealing one box, it may mean one label. If you are dealing with many boxes, it may mean labels in general.

Chinese is often less explicit than English about number.

The comma helps separate the sentence into two parts:

• 封箱以前写标签
• 以免拿错

This makes the structure easier to read:

1. first, the instruction
2. then, the purpose

So the sentence is organized like:

• Do X, so as to avoid Y

Chinese punctuation often helps show these logical chunks clearly, especially in short written instructions.

It is usually stronger than simply close.

封箱 often means:

• seal up the box
• close and seal the box for storage or shipping

So it suggests a more final action than just casually shutting a lid.

That is why 封箱以前 makes sense in a packing/shipping context: once the box is sealed, it is less convenient to label or check it.

It most naturally sounds like an instruction or reminder.

Because there is no subject and the verbs are in the plain form, it has the feel of:

• a packing rule
• a note on a workplace sign
• a procedural step
• advice

In English, you might naturally translate the tone as:

• Write labels before sealing the boxes, so you don’t take the wrong one.
• Label boxes before sealing them to avoid mix-ups.

So grammatically it is a simple sentence, but pragmatically it sounds like guidance.