First of all, let me clarify the meaning of the grammatical subject. The subject is what is performing the action onto the object. In the sentence "I do homework", "I" would be the subject and "homework" would be the object. Now for your original question. If you want to have multiple subjects performing the same action, you can use 하고 to mean "and" between the two subjects, and append the subject marker after the second subject. For example, John씨*하고* Bob씨*가* 한국어를 배우고 있어요. John and Bob are learning Korean. This would be the answer to your question in the title. Having said that, however, it seems that you're really trying to ask another question. You asked how you should place the subject marker after "today" and "I". I'd like to explain that "today" in your example sentence is not a subject. Rather, it is an adverb describing the action of you studying because it specifies the time. After time adverbs, we add 에 in Korean (it can be left out sometimes). So the Korean translations for your sentences would be: 오늘(에) 제가 한국어를 3시간 동안 공부했어요 and 저회가 어제(에) 노래방에 갔어요. Please let me know if you are still confused in the comments. Good luck :)!

We usually put subject first then object, at the end of the sentence, verb. however, we sometimes omit subject, object and verb sometimes. It will be really hard for especially English speakers to learn Korean, so you just don't stick to your English habit, then it will be much easier. I will show you some example sentence. From your stated sentence, Today I studied Korean for 3 hours and Yesterday we went to Karaoke Well, these sentences don't match the subject, so it is not clear who did what. And we don't know that 'who we refers' We have to know who we are, so you have to be more specific in Korean. So I will change 'we' to '친구들' instead of just '우리 - we'. 1. 어제 친구들과 노래방에 갔었고, 오늘은 한국어 공부를 세시간 동안 했다. 2. 친구들과 어제 노래방에 갔었고, 오늘은 한국어 공부를 세시간 동안 했다. 3. 친구들과 노래방에 어제 갔었고, 오늘은 한국어 공부를 세시간 동안 했다. 4. 어제는 친구들과 노래방에 갔었고, 오늘은 한국어 공부를 세시간 동안 했다. 5. 친구들과 어제는 노래방에 갔었고, 오늘은 한국어 공부를 세시간 동안 했다. 6. 친구들과 어제 노래방에 갔었고, 오늘 한국어 공부를 세시간 동안 했다. 7. 친구들과 어제 노래방에 갔었고, 한국어 공부를 오늘 세시간 동안 했다. 8. 친구들과 어제 노래방에 갔었고, 한국어 공부를 세시간 동안 오늘 했다. You may notice that I omit the subject '나/저, I' in the examples, because it is very obvious the subject is 'I'.