There are three simple tenses;

  • Simple present
  • Simple past
  • Simple future

If we look at the names of the verb's inflectional forms they are;

  • Present (go)
  • 3rd person singular (goes)
  • Past (went)
  • Past participle (gone)
  • Present participle (going)

I think most of problem is solved by just the names of the verb forms. It means present form is meant for present tenses and past for past tenses. From the names of verbs we get to know that there is no verb which stands for future. Verbs in English don't stand for future whereas that may be in some other languages. Since we have to make future sentences so the modal operators will and shall are used along with the main verb.

The auxiliary [do] is used in the simple present tense, and in accordance with the above formula regarding the use of verbs in various time situations the use of auxiliaries in different tenses will be as follows;

Tense Auxiliary used
Simple present do (with I, we, you, they and plural noun)
does (with he, she, it, singular noun)
Note: Both auxiliaries are usually used in negative or interrogative sentences. Their use in affirmative sentences indicates emphasis.
Simple past did
Note: it is usually used in negative or interrogative sentences. Its use in affirmative sentences indicates emphasis.
Simple future Shall (with I and we)
Will (with all other subjects)
Note: Will is mostly used now-a-days even with I and we. When theyare exchanged they show emphasis.

Starting from simple present [does] is the only 3rd person singular form used in present time situations. In case of affirmative sentence where does is not used still the main verb is in 3rd person singular form, like;

  • He considers me.
  • He doesn't consider me.

The use of do, does and did are mostly restricted in the negative and interrogative sentences and not used in affirmative cases, besides when does is used the main verb shall be converted to present form. Similarly with did there shall be present form instead of past. And when the speaker tries to show emphasis these auxiliaries are used in affirmative sentences too.

Examples

Simple affirmative sentences

  • I understand what you are talking about.
    He understands what you are talking about.
    I understood what you were talking about.

Emphatic affirmative sentences

  • I do understand what you are talking about.
    He does understand what you are talking about.
    I did understand what you were talking about.

 

Negative sentences

  • I don't understand what you are talking about.
    He doesn't understand what you are talking about.
    I didn't understand what you are talking about.

In simple future tense auxiliaries will or shall are used. It is already told in the note of above table that shall is used with 1st person pronouns (I and we), but we see that mostly it is not the case especially in American English where will is used with every subject. Anyway the exchange of auxiliaries show emphasis or determination too and shall is mostly used for that purpose.

The used of shall

Following are the uses of shall. Read them carefully.

1. To express determination on the part of speaker as while giving threats it is said,

  • You shall pay for this.

2. It is used when the speaker wants to show certainty.

  • Good days shall come.

3. To show compulsion.

  • You shall be here by 8 o'clock tomorrow.

4. To show uncertainty for future.