Stem + ます (Polite Form)
The Stem of a verb is seldom used in isolation. When it is used alone, it is used as a noun. Most of the time, the Stem is followed by another element. We form a polite form by changing the dictionary form of a verb to the Stem and by adding ます to the Stem. What you need to know is the conjugation of the Stem.
How to derive the Stem:
- Group 1 verb: drop る of the dictionary form
- Group 2 verb: change the final vowel u to i
- Group 3 verb: くる→き、する→し
Exceptions: the ending of the Stem of the five honorific verbs should be い rather than り （いらっしゃい "be/go/come", おっしゃい "say", ください "give me", さない "do", ござい "be") when ます is added.
How to use the form:
The Polite Form is created by adding ます to the Stem in all cases. For instance, the Stem of the verb 行く（いく）, which means 'to go,' is 行き, so the polite form is 行きます. This newly-created polite form of the verb has the following five aspectual forms. Notice that the changes occur only on the ending, ます.
'I will go to Japan.'
'I went to Japan.'
'I (normally) go to Japan.'
'I will not go to Japan.'
'I did not go to Japan.'
'I (normally) do not go to Japan.'
'Let's go to Japan.'