Not sure what you mean with "Sharepoint form", but here's how you can add
"created" and "last modified" fields on an InfoPath form:

Created:
- Add a field to your form to save the date & time in when the form is opened.
- Go to Tools > Form Options... > Open and Save tab
- Click on the Rules... button
- Add a rule to set the value of the field to the now() function using a
formula.
- Add a condition to this rule to only apply the rule when the field is blank.

Last modified:
- Perform the same steps as with the "created" field, but leave out the
condition on the rule.

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S.Y.M. Wong-A-Ton