Translation:This student has to overcome her laziness.
I wrote "This student has to overcome their laziness," using their because I didn't know the gender of the student. Is học sinh feminine? Is there something else about the sentence that makes the gender of the student clear?
This student can be male or female because there is nothing in the sentence make the gender of the student clear. So both his laziness and her laziness are accepted.
I'm not sure about using "their" here because this sentence mentions only one student ( this student).
"This student can be male or female because there is nothing in the sentence make the gender of the student clear."
"They" can used in English when you don't know whether a someone is male or female. In this sentence, "their" would be much more natural than "his" or "her" if there is one student and you don't know if they're a "he" or a "she".
I have also understood "they" as an acceptable singular pronoun when the person's gender is unknown.
Actually, although 'they' is generally used in common speech it's not grammatically correct. It needs to be 'his laziness', or 'her laziness'.