A commonly-used variation inserts a IV7 chord in bar 2…
… which provides a short excursion away from the I and adds more harmonic interest and motion than the otherwise static harmony in the first four bars…
Play along and sing the roots of each chord out loud (Hint: C=Do, F=Fa, G=So). Doing so is an extremely effective and efficient way to do ear training.
Frank playing a real-life example based on the form above…