It is already in its base form, which is the infinitive form.
Here are some examples of how it can be used:
* Present tense: I conclude that the meeting was successful.
* Past tense: She concluded her speech with a powerful statement.
* Future tense: We will conclude the project by the end of the week.
* Present participle: They are concluding their negotiations.
* Past participle: The agreement has been concluded.