The form -고 is the way how you can connect two or more sentences. That way you can list two or more facts, actions, states, etc. It is a simple form to make a list of things. However, you can also use it to express a sequence of actions where one happens after the other. All in all a pretty nifty, basic form.
The first use of -고, is the use everyone knows about. You use it to connect two or more sentences and form one larger sentence. Doing so creates a list of two or more facts, actions, states, etc. In this regard, it is pretty easy to understand Korean grammar form as it is no different from the English word ‘and’. Furthermore, the connective adverb 그리고 serves the same purpose as the form -고. You can use either of the two without any change in meaning.
가: 내일 날씨가 어떨까요?
나: 비가 오고 바람이 많이 불 거예요.
A: What will the weather be like tomorrow?
B: It will rain and there will be a lot of wind.
However, -고 has another meaning. Through -고 you can express that one action happened after the other. In this case, you tell in which sequence various actions took place. When you use -고 to express the sequence of actions, it is actually the shortened version of the form -고 나서. However, unlike -고 나서, it does not explicitly indicate the first action is completely done.
가: 내일 뭘 할래요?
나: 친구를 만나고 쇼핑을 같이 할 거예요.
A: What are you going to do tomorrow?
B: I am going to meet a friend and then go shopping together.
How To Use
Like for its friends -지만 and -거나, the conjugation of -고 is among the easiest you will find in Korean. It follows the “no rule” conjugation rule. Thus you simply need to take the verb stem and attach the ending -고 to it. After that, you are done. You needn’t worry over Korean irregular verbs when us it either.
It is not wrong to use tense markers, like the past tense -았/었-, with -고. However, when you use it to express a sequence of actions, you cannot. The tense is expressed only in the final verb as already stated in this article.
Lastly, the form -고 is meant for verbs and adjectives only. You cannot use it with nouns. For nouns, you will have to use a particle. You can use either the particle 와/과, 랑/이랑 or 하고.