Can we say that iv prepares the V in a minor key? Because iv is a subdominant chord and V is the dominant?
Your question is a little bit unclear, but yes: generally speaking, the subdominant chord (IV or iv) prepares the dominant (V).
(In the answer below, the Roman numerals should be understood in both major or minor. When I say IV, I mean IV in major or iv in minor. The only exception is V, which must be major [V, not v] to create dominant function.)
To generalize this a bit more, we say that the IV chord has "predominant" function. This is exactly what it sounds like: by "predominant," we're literally saying it comes before ("pre") the dominant.
The ii chord also has very clear predominant function. Less often, the vi can function as predominant, and in rare cases so can the iii chord.
Lastly, the IV chord doesn't have to be a predominant. It can resolve back to tonic, and in popular music, it's very common for it to come after the dominant!
I would say that it's a bit the other around. At least classical or more accurately CPP composers tend to establish three main harmonic areas. The "tonic" is the harmonic area of rest or finality or even beginning. The "dominant" harmonic area tends to signal the approach of the tonic area (though event may be evaded for artistic reasons.) The "predominant" harmonic area tends to suggest harmonic movement towards dominant harmony. The boundaries between these areas are not necessarily sharp. A 4-bar phrase with the chords I-I-ii6-V has clearly marked boundaries. However even something like I-vi-ii6-V the vi chord can be seen as an extension of the tonic or the beginning of the predominant area (or as I think of it, a "passing" chord.) In I-ii6-I64-V7-I. I I64 can be considered a predominant harmony leading smoothly to V7 (especially in concertos where everyone pauses on the I64 and the soloist plays a bunch of showy stuff) or just a decoration to the V7.
In cases like I-IV6-I6 or I6-IV6-I, the IV6 really is a passing chord (or neighbor chord, it has no "function" as in functional analysis.) Similarly in the Folia progression, i-V-i-VII-III-VII-i-V-i the first i-V-i isn't a cadence (or rarely; one could lengthen the time of the second i, i-V-i-i, and make the V-i into a cadence.) The final V-i is often treated cadentialy even appearing as i-V-i-VII-III-i-V-I.
Music is generally recursive in that one could (somewhat like Schenker or the generative music guys) take a pattern like I-V-I expand each part. One can always repeat a chord or insert that chord's dominant chord (like I-V-I becomming I-I-V/V-V-I) where V/V is the "analysis name" for II. In the "large" one can insert a ii-V belonging to a chord before the chord. Expanding this pattern usually leads to a predominant pattern followed by a dominant leading to the tonic (at a higher level, this may only be a phrase). Any I digress and risk turning the idea of harmonic regions into a durchkomperniert ramble (perhaps I should uses Omani musical patterns and create a Muscat Ramble).
In the major/minor system chord
V are the primary chords. (In minor you can generally use symbols
V.) These are the primary chords. In very loose terms you can play these chords in any order and produce a clear sense of tonality.
IV is one of the tonal pillars.
In functional harmony there is a sort of norm for harmonic flow moving pre-dominant to dominant to tonic. Roughly speaking that would be
? V I where
? is any chord not a dominant or tonic. Lots of chords can fill that pre-dominant role and
IV is one of the most common.
However, that doesn't mean that
IV always goes to
V or that
V must be preceded by a pre-dominant. Pre-dominant to dominant to tonic is just a norm, not the only option.
One way that
IV is found in a different functional role is in prolongation of some chord.
I IV I is a tonic prolongation and
V IV V just prolongs the dominant.
does the subdominant prepare the dominant : a general rule?
Not as an exclusive rule.
It is one of the most common roles of
IV, but not the only role.
First thing - if we're talking in terms of functional harmony, dominants, subdominants and tonics, it's likely to be V (major) not v (minor) even in a minor key.
(OK, I see you've edited that in your question.)
Yes, iv fits nicely before V. We can explain this by noticing that iv is closely related to ii7. And ii7, V, i is a standard 'cycle of 5ths' progression.
Let's shift over to an actual instance. I'm not sure SE can cope with the symbols I need to make this clear usind functional chord names.
In D minor I'm talking about Em7♭5 going to A7 going to Dm. Em7♭5 is E, G, B♭, G. The root notes E, A, D are 'cycle of 5ths'. Gm - that's G, B♭, D contains most of the notes of Em7♭5 and therefore can do the same job.
'v is the dominant'. Not necessarily. V can be and is more often the dominant.
But that's not the point. It's called subdominant because it's under the dominant. It has no particular role with relation to the dominant. Sometimes it precedes the dominant, sometimes it doesn't, and other chords do instead.
So, basically, no. In fact, if the 'dominant' is v, it's considerably weaker than if it was V, and as such, probably has no more role to play than iv itself.
Chords that do precede the dominant are likely to be iii, V/V or i itself.