Past:   

"I walked to school today"

This sentence describes something that happened in the past.

Present:

 "I am walking to school today"

This sentence describes something that is happening right now.

Future:

"I will walk to school today" 

 This sentence describes something that is going to happen in the future.

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And in your introduction there, it would be; (I) Hope you are all doing good.

The "are" would be present, how we all currently are. In past it would be 'I hope you all were allright (or good, or did good)' 

And in future tense it would be 'I hope you all do good (or well)'

hi dear

the past simple is used to refer to :

1- completed actions or events thet happened at a particulat time in the past

2- completed actions or events ehich took place over a specified length of time in the past

3- actions that were habitual or repeated over a period of time