Learn Hardcore Chinese: tā jīntiān zǒucuòlù le, suǒyǐ lái de hěn wǎn. - He went the wrong way today, so he arrived very late.

了 here is the perfective aspect marker. It shows that the action taking the wrong road is viewed as a completed event.
It often corresponds to past events in English, but it is not a true past-tense marker; it just marks a bounded, finished action.
Without 了, 他今天走错路 could sound more like a general statement or might feel incomplete in this context, because the lateness is clearly caused by a specific finished event.

Why is 走错路 written as three characters? What exactly does 错 do?

走错路 is a verb–result–object structure:

走 – to walk / to go (by walking)
错 – wrong; here it’s a result complement (showing the result of the action)
路 – road / way

So literally it is “walk–wrong–road” → to take the wrong road / go the wrong way.
错 tells you how the action turned out (wrongly).

Could we say 他今天走错了路 or 他今天走错了 instead? What’s the difference from 他今天走错路了?

All three are possible, but with small differences in focus:

他今天走错路了 – natural, common; treats 走错路 as a set phrase “take the wrong road”, and 了 marks completion.
他今天走错了路 – puts a bit more attention on 路 as the object: “He took the wrong road today.” Slightly more emphasis on the road.
他今天走错了 – leaves out the object, relying on context. It just says “He went the wrong way today” without specifying 路, which is understood from the situation.

In most everyday contexts, 走错路了 is the most typical.

How does Chinese show cause and effect here? What does 所以 do?

所以 means “so / therefore” and introduces the result clause.
The structure here is:

Cause: 他今天走错路了，
Result: 所以来得很晚。

So the sentence is literally: “He today took the wrong road, therefore came very late.”
You could also make it an explicit 因为…所以… pattern:
因为他今天走错路了，所以来得很晚。

What does 得 do in 来得很晚? Why can’t we just say 来很晚?

得 here is used to introduce a degree complement:

来 + 得 + 很晚 → “came (to the degree of) very late.”

The pattern is: Verb + 得 + (degree / manner / result).
You cannot normally say 来很晚 on its own; that sounds ungrammatical. You need 得 to link the verb 来 to the descriptive phrase 很晚.

What is the function of 很 in 很晚? Does it always mean “very”?

In predicate position, adjectives in Chinese usually need an adverb such as 很, 非常, etc.
Here, 很 serves two roles at once:

It often acts as a neutral linker so that 晚 can function as a predicate (“(was) late”).
It can also keep some of its literal meaning “very / quite,” depending on context and intonation.

In 来得很晚, it usually sounds like “quite late / pretty late”, but the emphasis is often not as strong as English “very late.”

Is there a difference between 来得很晚 and 来得晚?

Both are grammatical:

来得晚 – “came late.” Fairly neutral.
来得很晚 – “came quite late / very late,” with more emphasis on how late it was.

Native speakers tend to use 很 a lot in spoken Chinese, so 来得很晚 feels more natural and conversational, and also matches the idea that his lateness was noticeable.

Where can 今天 go in the sentence? Could we say 今天他走错路了?

Yes. 今天 (time word) is quite flexible. Common options include:

他今天走错路了，所以来得很晚。
今天他走错路了，所以来得很晚。
他走错路了，今天所以来得很晚。 – this last one is odd; 今天 normally goes before the verb clause it modifies.

Most natural are:

他今天走错路了，所以来得很晚。
今天他走错路了，所以来得很晚。

Both are fine; putting 今天 right after 他 is very typical.

Do we need to repeat 他 in the second clause, like 所以他来得很晚?

You don’t have to. Chinese often omits repeated subjects when it’s clear from context.

他今天走错路了，所以来得很晚。 – natural; 来得很晚 clearly refers to 他.
他今天走错路了，所以他来得很晚。 – also correct; adds a tiny bit of emphasis on he as the one who was late.

Both versions are acceptable; the version without the second 他 is slightly more concise.

How do I know when to use 的, 得, and 地? Why is it 得 here?

Rough rule of thumb:

的 – usually links noun modifiers to nouns: 漂亮的衣服 (pretty clothes).
地 – usually links adverbs to verbs: 高兴地说 (happily said).
得 – introduces complements after verbs or adjectives: 跑得快 (runs fast), 说得很好 (speaks very well).

In 来得很晚, 得 is correct because 很晚 is a degree complement describing how he came (how late). So it must be 得, not 的 or 地.

Does this sentence imply that he normally isn’t late, but today he was late because of the wrong road?

Yes, that’s a natural inference.
The structure “(Today) X happened, so (he) came very late” suggests that his lateness is specifically caused by today’s event (走错路).
It doesn’t state his usual habit, but contextually it feels like an exception rather than “he always arrives late because he always takes the wrong road.”

Could we replace 走错路 with 迷路? What’s the nuance difference?

You could say:

他今天迷路了，所以来得很晚。

迷路 means “to get lost” in general (lose one’s way).
走错路 is literally “take the wrong road”, which can be more specific and a bit milder: maybe he just took a wrong turn, not completely lost.
In many everyday situations, both can explain why someone was late, but 迷路 feels like more serious or complete confusion about where to go, while 走错路 can be a simpler mistake.