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I'm learning jazz and playing from lead sheets and am confused about how to handle triads that do not explicitly say what to do with the 7th. If the lead sheet I'm playing from just describes a chord as G, what are my guide tones for that? If I decide to just throw in a minor or major 7th, am I changing the function of the chord?
A plain triad sounds a lot different and either a major or a dominant chord, so I've been almost thinking of them as their own kind of chord.
A basic, simple 'rule' is that we have Ⅰ, ⅱ, ⅲ, Ⅳ, Ⅴ, ⅵ, ⅶ° as the main triads diatonically.
The 7ths associated with those are as follows: ▵, m7, m7, ▵, dom 7, m7, °7.
When Ⅰ moves directly to Ⅳ, dom7 is appropriate. But above all, use ears. They will tell, in 99% of cases, whether the choice is good or not.
Direct answers
Guide tones:
Major and minor triads generally are not thought of as having guide tones in the way seventh chords are, since they are considered stable within themselves; whereas seventh chords are considered inherently unstable and needing to resolve. The guide tones are the notes that most typically are the primary points of resolution: the third and the seventh.
To the degree there is (contextual) tension in a major or minor triad, the third of the chord is the first place to look, as it will often be the leading tone of the following chord.
Does adding a seventh change the function of a chord?
One should proceed as though it will. Turning a major triad into a dominant seventh, for example, potentially signals a modulation. It might or might now work depending on the musical context. Similarly, turning a dominant V chord into a major seventh weakens its function as a dominant chord.
Triads and seventh chords as separate entities
Music theory considers triads and seventh chords as separate entities. Further extensions (9ths, 11ths, 13ths) are generally interpreted in terms of the underlying seventh chord.
Detail
When a score calls for a triad, it should be played as a triad, without a seventh. Adding the seventh changes the sound and function of chords and risks clashing with the melody.
It is often okay to add a sixth to a triad, for color, because adding a sixth doesn't change the nature of the chord in the way a seventh does.
If the performer judges that a seventh can be played where a triad is specified, then one can follow @Tim's guidelines for major keys and the following for minor keys:
Minor key seventh chords i = m7 (or mM7 if the seventh is raised) iio = 7b5 (half-diminished) III = M7 iv = m7 v = m7 (or V7 if used as a dominant chord / with raised seventh) VI = M7 VII = 7 (i.e., dominant seventh)
Be careful, when adding sevenths, that you're aware of any modulations. For example, if one is in the key of C major, but encounters the triads F and Bb at a cadence point, then the minor seventh would be added to the F chord — creating an F7, the dominant of Bb — rather than the major seventh, which would otherwise be consistent with C major.
In short: know what the simple written version would sound like if played literally like that, and then do whatever sounds good to you. You must develop your own sense and taste for what sounds good through practicing and trying things. Theory gives you ideas and tools for finding things to try, and to reason about what others play.
We could solve this question either by writing an endlessly long list of logical rules and checklists that even a deaf person could mechanically apply... or we could solve it like it's actually done: with ear and taste.
The only actual rule is: play whatever supports the melody appropriately for the occasion. If it makes you feel better, play it. If it makes your audience feel better, play it. If it makes you or your co-musicians confused, don't play it.
Chord symbols are written (by people) as guidance for accompanists (other people), and together with the lead melody, they describe the lead-sheet writer's view of what the most important idea of the song is, when described in simple, coarse terms. But it's just a starting point - all that can (and should) be jazzed up in various ways. Whatever chord symbol is written, it's just one possible idea, and each chord symbol should be read as meaning a rough umbrella term as a whole family of possible chords. And even then, you can ditch the whole rough idea and play something from a different family of chords, if you think that it suits your own idea better.
Quote from Jazz Piano Book by Mark Levine
The unhappy fact that the chord symbols C, C∆, Cmaj7, CM7, C6, and C69 all mean pretty much the same thing and are often used interchangeably can be discouraging to a beginner. In this book, I’ll use the ∆ symbol for all major chords.
So, in his book, he writes a "maj7" chord symbol, whenever he means any sort of non-dominant major chord. The actual realization of the described idea (IF you decide to follow that idea and not some other one) can even be a simple triad, or it could be a four-note or five-note chord, and it still fits the same general category. But dominant chords are a different family of roles, as you know.
However, lead sheets are not generally written like that - Levine just wants to say with this notation that many types of chords are interchangeable, and when you see any written chord symbol, you should generalize it in your mind to a wider overall chord family, and then play something within that general ballpark. You learn to do that by trying different things and messing with the songs.
Your example lead sheet of My Favorite Things, starts with four consecutive bars of "Em". It certanly cannot mean that you must hold the exact same chord for four bars, that would be boring, wouldn't it? Myself, I would be tempted to play a descending line: Em - Em maj7 - Em7 - Em6 - ... and on the next line, instead of the written Cmaj7, I would do the same over Am: Am - Am maj7 - Am7 - Am6 (depending on the occasion, maybe trying not to sound like Stairway to Heaven). But then there would have to be yet another change on the third line to get a change there... Maybe if the second line has to be a C based chord, I might alternate between Cmaj7 and D/C, to keep some motion going.
what are my guide tones for that?
Your guide tones are from the chord that you imagine. You might see four bars of "Cmaj7" written on the lead sheet, but if you want to play: C - D/C - C - D/C, then over the imagined C your guide tones are from C, and over the imagined D/C your guide tones are from D/C.
If it says G it might truly mean a G triad. Especially if a country or pop tune.
If that doesn’t sound quite right then it probably means GMaj7 or G6.
If neither of those sounds right then you should look at the chords before and after it and figure it out from the context. So if it is followed by C7, chances are it is a Gmin7, or if it is followed by C7b9 it is probably Gmin7b5.
If it is a 12 bar blues then they probably mean G dominant 7.
