Use both the past and the imperfect.
1. Separate the 3 root letters.
2. Read the middle vowel in the past.
3. Find the imperfect or its symbol in a reliable dictionary.
4. Match the pair to one of the 6 pattern pairs.
A root may be used in more than one bab. Therefore, do not decide the bab from the meaning or past form alone. The same past form may occur with different imperfect vowels: this lesson shows حَسِبَ—يَحْسِبُ as the Bab Hasiba prototype and حَسِبَ—يَحْسَبُ under Bab Sami'a. Learn the complete pair from the dictionary.
Worked example: د خ ل + symbol ن gives دَخَلَ—يَدْخُلُ.