Step 1: Understanding the Verb Construction:
The sentence requires the infinitive form of the verb "understand" because it follows the adverb "properly." In English, when "properly" is used with a verb like "understand," we use the infinitive form "to understand."
Step 2: Analyzing the Options:
- Understand: This is the base form of the verb, but the sentence requires the infinitive.
- To understand: The correct choice, as it is the infinitive form used after "properly."
- Understood: This is the past tense form of the verb, which does not fit the context here.
- Understanding: This is the gerund form, which is incorrect in this sentence.
Step 3: Conclusion:
The correct form is to understand, as it fits with the verb construction following "properly."