Korean plain form is frequently used in writing and speech. In speech, it is casual statements to friends or talking to oneself. In writing, you will see in the new articles a lot. Let’s compare difference between (어/아)요 and casual statement to your friend.

나 어제 고등학교 친구 만났다/I met a high school friend yesterday.

나 어제 고등학교 친구 만났어(요)/ I met a high school friend yesterday.

They are exactly the same when a translator translates them. However, there is slightly different: the first one is unexpectedly happened and the speaker boasts what happened. Let’s see one more example.

나 조금 전에 아주 맛있는 초콜릿 먹었다. I ate much tasted chocolate a little while ago.

In the video, there are many examples you can see with past and future tense.