Question: I find when playing in a minor key I always end up playing the major version of the V chord. (So in Em I always end up playing a "B major" chord.)

However, the third of this chord should be played as a minor when looking at the key and it seems like using an accidental as frequently as I do is not really in the spirit of accidentals.

Am I making a mistake in my music theory? Am I playing the key I think I am? Is this just a mode? Do I need to start playing more B minors?

Please bear in mind I am relatively new to music theory and would appreciate it if you treat me like an idiot. Thanks.

Answer: 
Say we play in Em (relative major is G, one sharp). The V chord of Em should be Bm (B D F#). Although playing B major adds a sharp to D it is not considered as an "error" since it can naturaly lead to Em. It's clearer if you play B7 (B D# F# A) as it's the dominant chord of Em.

Sometimes the II chord of the major scale (Am in G) can also be played as a II7 to lead to the V chord. Even the I of the minor scale can be «dominant»-ized.

Try to play those progressions:

* G Bm Em Em Am D7 G G (nothing special)
* G B7 Em Em Am D7 G G (B7 leads to Em in a strong way)
* G B7 Em E7 Am D7 G G (same with E7 to Am)
* G B7 Em E7 A7 D7 G G (same with A7 to D7)
One way to look at things is that, for music in a minor key, the chord on scale step 5 is minor (Gm in the key of Cm). In a cadence, scale step is usually raised to be a half step below the tonic (scale step 8). In non-cadential passages, often the minor chord on the 5th step is used. For example, in a cycle of fifths, one might have (in the major): I,IV,vii0,iii,vi,ii,V,I etc. In a minor key, an ending cycle might look like: i,iv,VII,III,VI,ii0,V,i. However if the passage contains two cycles one often gets: i,iv,VII,III,VI,ii0,v,i,VII,III,VI,ii0,V,i. The "interior" part of the passage is just a string of chords, not a cadence. Of course, this treatment is optional and neither is incorrect; they are just different.

A similar example occurs in sequence decent by thirds (Pachelbel canon for one example). In a major key: I,V,vi,iii,IV,I6,ii6,V (where the descending bass turns around for the last two chords, not the only choice.) In minor keys the following is often found, i,v,VI,III,iv,i6,ii06,V in a cadential context; the last chord can be v if not meant cadentially. 

As an aside: this sequence is very flexible. In a minor key, the chords on the scale step 5 can be either major or minor; the iii chord can be a I64 (not really cadential or arpeggiated); the ii0 chord can be iv; the ii chord can be IV; the ii or ii0 chords can be in root position; and lastly the final V (or v) chord can be a vii06 leading to a I (or i) chord which can give a bass line that descends an octave.