To insert a database field into a form you first need to make sure you are in form design mode. You can insert any field from those already in the attached table or a field from another table.
If you based the form on an auto form, then you merely need to show fields that you have previously hidden. However, it could be that you want a field to appear more than once in the form. In this case you need to specifically insert the field.
If you created a new form and did not base it on an auto form, then it could be that you forgot to include a particular field or that you want some field to appear more than once. Again, in this case, you need to insert the field.
For information on how to insert a database field, see Inserting a database field.