Jun lives in korea.
correct: Jun lives in Korea.
Explanation: Proper noun - a word or group of words that is the name of a person or place and always begins with a capital letter.

Example: London, Sarah, Icecream Store, California, Science, Math.

Common noun- a noun that is the name of a group of similar things, such as 'table' or 'book', and not of a single person, place, or thing.

Todays temperature is a high temperature.
correct: The temperature today is high.
Explanation: The word temperature is redundant in your sentence.

Jun used a thermometer.

he is phsically gifted.
correct: He is physically gifted.
Explanation: You should start your sentence with capital letter.

jun measures.
correct: Jun measures the temperature.
Explanation: Your sentence "jun measure" doesn't give a complete Idea it is fragmented. Sentence - is a group of words, usually containing a verb, that expresses a complete idea. and the name "jun" should be written with a capital letter.

Thank you so much for submitting your assignment Jun. Review the corrections and practice writing in a complete sentence.

Remember:  "Practice makes perfect." Enjoy your Holiday Jun and see you in our next meeting. Take care.