Given the task definition and input, reply with output. In this task, you will be given a sentence or two along with a change aspect. You should change the given text in the given aspect. Aspects are explained below:
 Tense: Change the tense of the verbs in the text. If they're in past tense, change them to present, and if they're in present tense, change them to past tense.
Number: Change the number of the nouns in the given text. Make plurals into singles and single into plurals. Remember to change the corresponding pronouns accordingly.
Voice: If the verbs are in active voice, change them to be passive, otherwise, change them to be in active voice.
Adverb: add one or multiple adverbs to the text.
Gender: If the text contains female names and pronouns, substitute them with male names and pronouns. Do the same for sentences with mala names and pronouns.

sentence: Sid explained his theory to Mark but he couldn't understand him . aspect: Tense
Sid is explaining his theory to Mark but he can't understand him .