EntireRow copy/paste won't stop

because the active sheet now is not sheet a anymore
maybe try like this:

Dim cel As Range
    Worksheets("sheet a").Activate
    For Each cel In Range("f2:f1000")
        If cel.Value = "y" Then
        cel.EntireRow.Copy
        Worksheets("sheet b").Activate
        ActiveSheet.Paste
        Application.CutCopyMode = False
        ActiveCell.Offset(1, 0).Select
        End If
       Worksheets("sheet a").Activate
    Next

FYI, it will show you sheet a and sheet b back n forth.

If you know what row to be pasted in sheet b, (the last blank row, for example)
the code can be modified.