If you are simply asking about the verb form - 'I study ' as opposed to 'He studies' - you have an answer from Jmat. Or are you asking about the difference between when to use the noun in the singular (study) or plural (studies)? 'A study' is a single piece of research or a single academic work. The plural is 'studies'. For example 'The researchers took part in a study about X. There have been many studies focusing on this recently.' We also use the word 'studies', always in the plural and with the possessive pronoun (my/your/his/her etc), to talk about someone's academic endeavours, such as the subjects they are studying and the courses they are taking. For example, 'Tell me about your studies' or 'I can't have a job because my studies take up so much time'.

'study' is a verb. "I will study a new language this year." 'studies' is a noun. "I am having trouble with my studies." -- Looks cold in your profile picture, I respect that :-)

"Studies" is only used in the third person singular in the present tense. Eg. I/You/We/They study... He/She studies...