tā shàngbān de shíhou yídìng yào dài yǎnjìng, bù dài jiù kàn bù qīngchu diànnǎo.

Here 上班的时候 literally means “the time when (he) is at work.”

• 上班 = to work / to be at work
• 时候 = time, moment
• 的 links what comes before it (上班) to 时候, turning 上班 into a kind of modifier for 时候.

So the structure is:
[verb/object phrase] + 的 + 时候 = “when … / the time when …”

You will almost always see 的 in this structure.
上班时候 without 的 is not natural standard Mandarin; it sounds incomplete or dialectal.

时候 and 时间 are related but not identical:

• 时候: a point or period in time, often used with “when …”
  
  • 他上班的时候 = when he is at work
• 时间: “time” in a more general, measurable sense (amount of time, time available)
  
  • 他上班的时间很长 = The time he spends working is long / He works long hours.

In your sentence, we mean “when he is at work”, not “the amount of time he works”, so 上班的时候 is correct.
上班的时间一定要戴眼镜 would sound like “During his working hours he must wear glasses” — possible, but a bit more formal and slightly different in nuance.

In Chinese, once the subject is clear, it is often omitted in following clauses if it stays the same.

The full “expanded” version would be:

• 他上班的时候，一定要戴眼镜，他不戴就（他）看不清楚电脑。

But that’s very repetitive in Chinese. So it’s shortened to:

• 他上班的时候，一定要戴眼镜，不戴就看不清楚电脑。

Listeners automatically understand that 不戴 and 看不清楚 still refer to 他.

• 要 alone can mean “need to / should / going to,” depending on context.
• 一定 means “certainly, definitely.”

Together, 一定要 adds a strong sense of necessity or obligation:

• 一定要戴眼镜 ≈ “must wear glasses / absolutely has to wear glasses.”

Rough comparison:

• 要戴眼镜 – needs to / should wear glasses (could sound like a rule or plan, but sometimes softer)
• 一定要戴眼镜 – really must wear glasses (stronger; there’s a clear reason/requirement)
• 得戴眼镜 – colloquial “have to wear glasses,” similar strength to 一定要 in many contexts
• 需要戴眼镜 – “needs to wear glasses” (a bit more neutral/clinical, like stating a condition)

In this sentence, 一定要 emphasizes that for him, wearing glasses at work is unavoidable.

These three often confuse learners:

• 戴: to wear/accessorize something on the head/face/hands (and some accessories)
  • 戴眼镜 (wear glasses)
  • 戴帽子 (wear a hat)
  • 戴手套 (wear gloves)
• 穿: to wear clothes/shoes
  • 穿衣服 (wear clothes)
  • 穿裤子 (wear pants)
  • 穿鞋 (wear shoes)
• 带: to bring/carry something with you
  • 带手机 (bring a phone)
  • 带雨伞 (bring an umbrella)

So 戴眼镜 is correct because glasses are something you “put on” your face like an accessory, not clothes you 穿, and here we aren’t talking about bringing them along (带), but wearing them.

Yes, it’s very natural to omit repeated information in Chinese when it’s obvious from context.

• First part: 一定要戴眼镜 – wear glasses
• Second part: 不戴就看不清楚电脑 – if (he) doesn’t wear (them)…

The object 眼镜 is understood from the previous clause, so it doesn’t have to be repeated. Repeating it:

• 不戴眼镜就看不清楚电脑

is also correct, just a bit heavier in sound. Everyday speech often drops the repeat.

就 here signals a direct result or consequence: “if/when not wearing (them), then (he) can’t see clearly.”

• 不戴就看不清楚电脑 ≈ “If he doesn’t wear (them), then he can’t see the computer clearly.”

Alternatives:

• 不戴看不清楚电脑 – understandable, but sounds a bit abrupt or less natural; adding 就 smooths the cause–effect.
• 如果不戴，就看不清楚电脑 – also natural, and more explicit:
  
  • 如果 = “if”
  • 就 still marks the result.

Native speakers often omit 如果 when the conditional meaning is obvious and just use 就 to connect the condition and result, as in your sentence.

看不清楚 is a verb + potential complement:

• 看 = to see / look
• 清楚 = clearly
• 看清楚 = to see clearly
• 看不清楚 = cannot see clearly (unable to achieve the result “clear sight”)

This V + 不 + result pattern shows inability or failure to reach a result:

• 听懂 (hear and understand) → 听不懂 (can’t understand by hearing)
• 看见 (see) → 看不见 (can’t see)
• 看清楚 (see clearly) → 看不清楚 (can’t see clearly)

清 by itself means “clear,” but 清楚 is the common word for “clear / clearly” in this context. 看不清 is possible in speech but feels more clipped; 看不清楚 is more standard and natural.

In this sentence, 不 is used because:

1. 不 + potential/result complement
   With structures like 看得清楚 / 看不清楚, we use 不 to indicate (in)ability:

   • 看得清楚 – able to see clearly
   • 看不清楚 – not able to see clearly

2. 不 for general/typical cases or conditions
   不戴就看不清楚电脑 expresses a general condition: “if (he) doesn’t wear (them), (then) he can’t see.” For general truths, habits, or conditions, we use 不, not 没.

没 is more for past events, existence, or completion:

• 他没戴眼镜 – He didn’t wear glasses (on that occasion).
  Using 没看清楚 is also possible, but it usually refers to a specific past failure:
• 刚才我没看清楚。 – I just didn’t see clearly (that time).

In your sentence, we’re talking about a general ongoing situation, so 不 is correct.

In this context, 电脑 is used generically (“the computer / computer screen”) rather than counting individual items.

• 看不清楚电脑 ≈ “cannot see the computer clearly.”

Using 一个电脑 would sound like you’re talking about one particular, countable physical computer in a counting sense, which is unnecessary here and a bit odd.

Compare:

• 看不清楚电脑 – can’t see the computer (screen) clearly (general condition).
• 看不清楚那台电脑 – can’t see that computer clearly (specific one; 台 is the usual measure word for machines).

So in your sentence, no measure word is needed.

Both are possible:

• 他上班的时候，一定要戴眼镜…
• 他在上班的时候，一定要戴眼镜…

在 here explicitly marks “during (the time of) …”:

• 在上班的时候 = “during the time of working / while at work.”

However, 上班的时候 without 在 is already very natural and common. Adding 在 can sound a bit more formal or slightly heavier, but it’s not wrong.

For everyday speech and standard writing, 他上班的时候 is perfectly fine and probably more common.

No, that word order doesn’t work in Mandarin.

The typical pattern is:

• [subject] + [time phrase] + [rest of the sentence]
• Time phrases are often formed with …的时候.

So:

• 他上班的时候，一定要戴眼镜。 ✅
  “When he is at work, he must wear glasses.”

Putting 的时候 after 戴眼镜 as 戴眼镜的时候上班 changes the meaning and still sounds unnatural here. A different, correct sentence would be:

• 他戴眼镜的时候，看得很清楚。
  “When he wears glasses, he can see clearly.”

But you can’t flip it around as in English: you don’t say “he must wear glasses when work.” The …的时候 phrase has to directly follow what it is describing (上班 / 戴眼镜 / 他), not float around freely.

To express “must not wear glasses”, you have a few options, with different nuances:

1. 他上班的时候，不能戴眼镜。

   • 不能 = not allowed to / can’t
     Strong prohibition or impossibility (e.g., safety rule, physical restriction).

2. 他上班的时候，不可以戴眼镜。

   • Similar to 不能, explicitly “not allowed to.”

3. 他上班的时候，不用戴眼镜。

   • 不用 = doesn’t need to
     Means “doesn’t need to wear glasses,” not “must not,” so weaker.

4. 他上班的时候，一定不要戴眼镜。

   • This can mean “definitely don’t wear glasses (you must avoid wearing them).”
     It’s stronger than a simple “don’t,” but in some contexts 一定不要 can sound like strong advice or warning, not necessarily a formal rule.

If you want a clear, rule-like “must not”, 不能戴眼镜 or 不可以戴眼镜 is usually the safest choice.