Teach Us, Teacher
NAI-form verb + NAKEREBA NARIMASEN (Lesson 36)When you say you must or need to do something, you replace the NAI part at the end of the NAI-form of verbs with NAKEREBA NARIMASEN.
For example, if you want to say "I must study," you first change the verb BENKYÔ SHIMASU (to study) to its NAI-form, BENKYÔ SHINAI. Then, you replace NAI by NAKEREBA NARIMASEN. So, you say BENKYÔ SHINAKEREBA NARIMASEN. In a casual way, NAKEREBA NARIMASEN (must, have to, need to) becomes NAKYA. So, you say BENKYÔ SHINAKYA.
If you don’t have to do something, you change the NAI part to “don’t have to,” that is, NAKUTEMO II DESU. So, when you say “I don’t have to study,” you change NAI in BENKYÔ SHINAI (not to study) to NAKUTEMO II DESU, and say BENKYÔ SHINAKUTEMO II DESU.
But everybody! I tell you it’s important to study.