Alongside the topic particle 는/은 discussed in the last lesson there is the (grammatical) subject particle.

For the subject particle you have to attach -i 이 or -ga 가 (if the preceding letter is a vowel):

seonsaengnim-i 선생님이 (teacher)
maekju-ga 맥주가 (beer)

The subject particle is different from the topic particle in that the topic particle is used to make a general or factual statement whereas the subject particle is used to name specific things, usually ones that have been mentioned before and the speakers are aware of.

선생님이 느리다 = (That) teacher is slow

To state that teachers are slow in general you would use the topic particle 는/은.

The use of the two particles is similar to the way articles are used in English.

An apple is red = 사과는 빨갛다 (A general or factual statement about an apple)
The apple is red = 사과가 빨갛다 (A particular apple that the speaker identifies and indicates to the listener)