Lesson 3 - Identifier Particles - 이 / 가

가/이 is employed equally as 는/은 therein they indicate a theme however 가/이 is employed once it's necessary to spot the person or issue that's talked regarding in an exceedingly sentence.
For example,
나는 샀다 = I bought
내가 샀다 = I bought

Note: 나 is modified to 내 once it's used before 가.
Both of those 2 sentences mean "I bought" however 내가 샀다 puts a lot of stress on the topic of the sentence, "I".In the sentence, 내가 샀다, it's a lot of involved regarding 'who' bought instead of 'what' I did.
On the opposite hand, 나는 샀다, is a lot of involved regarding 'what' I did.
내가 샀다 = I bought [It wasn't anyone else however I WHO bought]
나는 샀다 = I bought [I bought instead of doing one thing else]

It is almost like the method in English wherever an individual stresses a precise word to grant it a lot of stress or importance.
For example,
WHO bought a brand new t-shirt?
내가 샀어 = I bought it.

Note: it's incorrect to mention, 나는 샀어, as a result of the person is asking regarding WHO bought a brand new T-shirt instead of asking regarding what they did.
What did you are doing within the park?
나는 잤어 = I slept (I took a nap)

Note: The person is also asking to many folks regarding what they did within the park. One person would possibly say they took a stroll whereas another person could have had lunch there. however as on behalf of me, what I did was 'sleeping'.
Therefore,
나는 잤어 = I slept

It is wrong to mention 내가 잤어 during this case as a result of the person isn't asking regarding WHO slept however what they did within the park. If the person asked Maine, 'Who slept within the park?', i might answer, 내가 잤어 or 마크가 잤어 (Mark slept).
The symbol particle, 가/이, identifies the topic of a sentence. 가/이 {is concerned|cares|thinks regarding|worries|is bothered} about WHO did the items asked.
Like the 는/은 example on top of, 가 is employed for words while not a final consonant and 이 for words with a final consonant.
Here ar example sentences:
내가 마셨다 = I drank (It wasn't anyone else however me)
동생이 먹었다 = Younger brother/sister Greek deity (It wasn't some other person however them)
민지가 왔다 = Min-ji came (It wasn't another person however Min-ji)
안토니가 갔다 = Anthony went (It wasn't another person however Anthony)
저것이 한강이야 = that's the Han dynasty watercourse
빌딩이 높다 = The building is high
공원이 넓다 = The park is large/spacious

마시다 = drink
동생 = younger brother/sister
먹다 = eat
민지 = Min-ji (a feminine name)
오다 = come
안토니 = Anthony
가다 = go
저것 = that
한강 = the Han dynasty watercourse (in Seoul)
빌딩 = a building
높다 = high
공원 = a park
넓다 = spacious, large