I wonder if this sentence is correct, and is there a better alternative for this sentence. The sentence is:
After the Japanese student graduate from the secondary school, he will have learnt 2000 Kanjis."
"Student" is a singular noun but "graduate" is a plural verb. You should either make the whole thing singular, "student graduates", or make the whole thing plural, "students graduate".
The article "the" in front of "student" makes the sentence refer to one specific student. I suspect you are talking about students in general. You could cast it in the singular and say "a Japanese student", or you could make it plural and say "Japanese students". That said, "the" is not necessarily wrong here. Sometimes English-speakers say "the student" to refer to a generic student. In a sense it violates the usual rules about "the" versus "a", but it's done fairly often.
I'd also guess that they don't learn the Kanjis AFTER they graduate, but rather they learn them while they are in school. So you should say "When a student graduates ..." or "By the time a student graduates ..."
So better phrasing would be:
By the time a Japanese student graduates from secondary school, he will have learned 2000 Kanjis.
When Japanese students graduate from secondary school, they have learned 2000 Kanjis.
A catch to using the plural is that in some cases it may not be clear whether you mean that each student learns 2000 Kanjis, or that all together the students learn 2000 Kanjis, i.e. if there are 200 students they may learn 10 each. I think in this case the reader would assume that you mean each student learns 2000, but it could be unclear in other contexts. Like, "When the volunteers completed the job, they were paid $1000." Was each volunteer paid $1000, or were all the volunteers combined paid $1000?
"When the Japanese students graduate from secondary school, they will have learnt 2000 Kanjis" - sentence talks about all the students graduating from secondary school in geneal.
"When the Japanese student graduates from secondary school, he will have learnt 2000 Kanjis." - sentence talks about a single student graduating from secondary school, gender of student not important here.
Does this help you??