Hint: If you create your relationships BEFORE creating your form and subform, Access will automatically create Master / Child link fields.
To do this, open up the Relationships GUI screen ("Tools" -> "Relationships") Add your tables. Example tblRob, tblProject. (What is frmEntry based on? You state tblRob but then reference tblRob is used for a subform???)
Then click on the primary key in one table and drag the key to the appropriate foreign key in the related table. For example, for a Contact database drag tblCompany.CompanyCode (primary key) to tblContact.CompanyCode (foreign key). At the popup window, check-off "Enforce referential integrity".
I am confused about your description...
a) - (frmEntry) that inputs data to a table (tblRob)
b) - (frmEntry) there is a subform (tblRob_subform)
c) - (tblRob_subform) should display the data in that table (tblRob)
d) - (tblProjects_subform) on the same form (frmEntry)
Are you suggesting that the main form is based on tblRob and is used for data entry? And the subform displays other information for tblRob.
How do you differentiate and group one tblRob data
This sounds like you may have a potential data sdesign issue. A common approach may be to have two tables such as Invoice and InvoiceDetail. When the Invoice "header" is selected, you can see / enter the Invoice detail records. This is a typical one-to-many relationship.
If I am right about the possible design issue, please review the following...
Fundamentals of Relational Database Design by Paul Litwin
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Micro$oft's answer...
Where to find information about designing a database in Microsoft Access
283878 - Description of the database normalization basics
304467 - ACC2000 Defining Relationships Between Tables in a Microsoft Access Database
Richard