Mariana, it would be extremely rare for you to conjugate a sentence with 'somebody' as the subject and 'explains' as the verb. If you did find such a sentence, it would more likely be in the past tense as in, "somebody explained to him how to understand women". Of course, that is fiction but it is still a good example. :)
I think the phrase you are asking about is actually in the command form as in, "somebody, please explain to me how to use this software!" In this case, "explain" has no "s" on the end. This would be a very common phrase using "somebody" and "explain".
I hope this helps.