I have some questions about this and my teacher is not understandable at all so I came here looking for answers.
I am supposed to write a bassline that has C Am F and G chords on a provided manuscript, but should I write the treble clef as well with the bass line or not? And can I use the 3 whole notes that are on top of each other or is it only for treble clef?
It sounds like your teacher has given you a chord progression ( C major, A minor. F major and G major). Possibly s/he has also given you a melody. It also seems that s/he has notated the four chords for you in the treble clef/staff so you know which notes you can choose from.
Your bass line would need to be written in the bass clef only.
It's not clear what rhythm is required. Perhaps it's one note per bar or perhaps you have to make one up for yourself.
The simplest bass line will just use the root note from each chord, so for C major the bass note could be C, for A minor the bass note could be A, and so on. Perhaps your assignment is more complicated than this, but if you write your bass line in this simple way then rush it in to your teacher for advice there is a chance things will go well.
I would be content with just writing out the bass clef with the chords written above each bar (ie as letters) for clarity.
not quite what you asked but also: a simple version of C Am F and G would "walk" between them, outlining each bar's chord so as a simple on the beat line
bar 1: C E G B(up)
bar 2: (down) A C G E (down)
bar 3: (up) F A C F (up)
bar 4: (up) G F D B(down) repeat
note that each final quarter note is just one scale tone away (harmonically and in pitch) from the start of the next bar so it "walks around".
I failed to answer one question of yours; you can use any note from the chord in your bassline, however be aware that some will sound better than others and affect the way the whole chord sounds (this is taking you into the realm of inversions). If you know a pianist or guitarist or can play guitar yourself, record a simple version of the chords and experiment with putting different notes in the bass under each chord.
