Morphology is the branch of linguistics that involves word forms.
In Toki Pona, words do not inflect: each word only has one form. It does not mark tense, aspect, number, or gender (either grammatical or on pronouns). While this is unlike English, plenty of natural languages do fine without this inflection, just as others have even more inflection that English speakers find unnecessary.
Information from inflections (such as tense, aspect, number, and gender) can be dropped if it is not important, or included explicitly if needed. Dropping this information is especially desirable for general statements, such as an observation that does not depend on tense.
Where necessary, modifiers are often used to specify number and gender. Preverbs and context phrases are often used for tense and aspect. Each strategy is flexible beyond these use cases, and allows other details to be discussed with the same level of importance.