Past or Perfect?

1. Two questions to ask
Are you telling what happened at a past time? Use the simple past. Are you saying how things stand now because of it? Use the perfect.

2. The simple past
It reports a finished event. Before an object or another word the verb lengthens its last vowel, and at the end of a clause it ends in -eɛ or -iɛ. See Simple Past Before an Object and Simple Past at the End of a Clause.

• Kofi baa ha nnɛra. (Kofi came here yesterday.)
• Medidiiɛ. (I ate.)

3. The perfect
A- says the event has happened and its result holds now.

• Kofi aba. (Kofi has come, so he is here.)
• Madidi. (I have eaten, so I am not hungry.)

4. States now
For how you feel or what you are now, Twi often uses the perfect where English uses the present: Mabrɛ (I am tired), Wawu (He has died, he is dead). See The Perfect for Present States. The past Mebrɛeɛ would mean you got tired at some past time.

5. Time words help
A finished past time such as nnɛra (yesterday) or afe a atwam (last year) goes naturally with the simple past. Words about now, such as seesei (now) or dada (already), go naturally with the perfect.

6. In the negative
The two tenses swap their markers: the negative past uses a- (Kofi amma, Kofi did not come), and the negative perfect uses the past-style ending (Kofi mmaeɛ, Kofi has not come). See Did Not and Has Not.