Subject Particle: 이/가

Simply put, 이 and 가 are used to identify the subject of a sentence. One situation where this participle is used is with adjectives: 

Looking at the two examples above, the participle "가" identifies the subject. In this case, is is the "head" that hurts, and it is a "map" that the speaker has. 

When used with a verb, it emphasizes the fact that the subject was the doer of the action (e.g. It was <subject> that <action>):

내가 줬어 is directly translated to "I gave", but it emphasizes the fact that it was "I", or the speaker that performed the action. Think of answering the question "Who gave it?". 

이 and 가 have the same function, but we use:

• 이 when the word before (object) ends in a consonant.
• 가 when the word before (object) ends in a vowel. 

To summarize, 이/가 are particles that identify the subject of a sentence. Most times, it gives an extra emphasis on the subject. Unlike 은/는, another subject particle, it is used more for questions and does not create a sense of comparison.