Here's why it's tricky to give an exact number:
* Many verbs have irregular stems in the imperfect, but the endings are regular. For example, "tener" (to have) has the stem "ten-" in the imperfect, but the endings (-ía, -ías, -ía, etc.) are the same as for regular verbs.
* Some verbs have irregularities in specific tenses, including the imperfect, but not in others. So, a verb might be irregular in the imperfect but follow regular patterns in other tenses.
* There are also variations within irregular groups. Some verbs might have irregular endings in the imperfect, while others might have irregularities in the stem.
Instead of focusing on a specific number, it's more useful to understand the patterns and groups of irregular verbs in the imperfect tense. This includes:
* Verbs with stem changes: These verbs have a vowel change in the stem in the imperfect, like "poder" (to be able) becomes "pod-ía".
* Verbs with irregular endings: These verbs have endings that differ from the regular pattern, like "ir" (to go) becomes "iba".
* Verbs with both stem changes and irregular endings: These verbs have a combination of both irregularities, like "saber" (to know) becomes "sab-ía".
It's important to learn and practice these irregular verbs, as they are frequently used in Spanish. Understanding the patterns and groups will make it easier to recognize and use them correctly.