The masu-form: polite verb sentences
The masu-form is a basic way to speak politely. Learn its affirmative, negative, past, and non-past forms separately so that you can read sentence endings accurately.
Build the form by verb group
For group 1, change the final u-row sound to its i-row counterpart and add ます: 読む → 読みます, 書く → 書きます, 買う → 買います. Do not attach ます directly to the dictionary form.
For group 2, remove る: 食べる → 食べます and 見る → 見ます. The irregular verbs are する → します and 来る → 来ます（きます）. Conjugation preserves the basic meanings of eating, seeing, and so on.
Compare four endings
読みます is non-past affirmative and 読みません is non-past negative. 読みました is past affirmative and 読みませんでした is past negative. Notice the でした in the negative past form.
昨日は読みませんでした。 means 'I did not read it yesterday.' 明日は読みません。 means 'I will not read it tomorrow.' Treating the masu-form as present tense only can lead to errors with future plans.
Separate politeness from meaning
毎日歩きます。 can describe a daily habit, while 明日歩きます。 can describe a plan for tomorrow. To say that you are walking at this moment, a progressive expression such as 今、歩いています is normally used.
Do not add です to make a polite verb sentence, as in the incorrect 読みますです. First choose the time reference and whether the statement is affirmative or negative, then use the appropriate masu-form ending.
Check: Which means 'I did not study yesterday': 勉強しません or 勉強しませんでした?
Show the answer and why
昨日は勉強しませんでした。 uses the past negative. しません is non-past negative; しませんでした is past negative.