Verb conjugation is the process of changing a verb to indicate something about it, obeying the grammatical rules of the specific language.
In language, a stem is the main part of the word; the part that holds the most meaning. To change the tense of the word, or to make it negative, parts are added onto the stem (for example, suffixes are added to the end). In Japanese, verbs need to be converted from their dictionary form into the stem before these changes are made.
How to form the stem
U-verbs: the stem is formed by replacing the -u at the end with an -i.
e.g. iku (to go) 行く→ iki 行き
Ru-verbs: the stem is formed by deleting the –ru at the end.
e.g. taberu (to eat) 食べる → tabe食べ
Irregular verbs: the two main irregular verbs are suru (to do) する and kuru (to come) くる
suru (to do) する → shi し
kuru (to come) くる → ki き
How to conjugate verbs
Stem + masu ます
Watashi wa Nihon ni ikimasu
I go to Japan.
Stem + mashita ました
Watashi wa Nihon ni ikimashita
I went to Japan.
Stem +masen ません
Watashi wa Nihon ni ikimasen
I don’t go to Japan.
Negative present + deshita でした
Watashi wa Nihon ni ikimasen deshita
I didn’t go to Japan.
*Any native Japanese speakers, please feel free to make any corrections. This information is based on studying from the Genki series of textbooks (Genki I, 2nd edition).*