When to use 了 and 的?

Question

了 is used after verb or sentence to indicate that something has been finished/done or something has changed.

的 is used in many cases, one of which is to indicate the speaker's modality of certainty.

他来了。 He has come. (O) -> 了 needs to be used.

他来的。 (X) -> 的 should not be used.

他现在才来了（X) -> Now, 了 should not be used in this case.

他现在才来的。He has just come. (O) -> Now, 的 needs to be used in this case.

Apparently there is some rule that I am missing, which makes me quite confused about how exactly these 了 and 的 work.

It will be really nice if I can tell which one is an appropriate one when I write things in Mandarin.

Thanks in advance.

Answer 1

他（是）现在才来的 is a 是的 construction for emphasis. In this case, 是 is omitted. He has arrived just now, in this very moment!

Another example: 我是来看病的: I have come to have a health check (rather than just socialize).

他来了 just indicates he has arrived, without emphasis. 了 completes the statement.

Answer 2

Textbooks should stop saying 的 forms past tense sentences. It does not. It forms adjectives. The grammar of 他现在才来的 is not like "he just now has-arrived" with the past tense of "to arrive." The grammar is "he is newly arrived," where "newly arrived" is an adjective modifying "he."

In fact they should stop talking about past tense in Chinese. Chinese is aspectual, not tensed. In those more accurate terms, 了 indicates perfect aspect of 来, while 的 indicates that 现在才来 applies to 他.

I defer to 倪阔乐's expertise in saying the pragmatic force of the adjectival form is to make some point explicit or emphatic. But I want to make the point that 了 indicates perfect aspect of a verb, while 的 links an adjective to a noun. They are quite different grammatically.

Both roles occur in the example : 你是怎么把他杀了的 from Ath's answer to your question 怎么.... with 的 and 了. There the 了 indicates perfect aspect of 杀 (did kill), while 的 indicates that 把他杀了 applies to 你.