Double sharps Bb clarinet

Okay. I'll give you the 'quick and dirty' version, but the long-term solution will be for you to pick up a first-year book on music theory and learn the basics from that. Note that what follows is very broad and most definitely not comprehensive, but it should give you enough to understand why a B-flat might sometimes be called an A-sharp.

'Modern' western music--everything after about the mid-1500s, that is to say--is based on major and minor scales, rather than the modes which were used in early Greek music, medieval plainchant, and music of the early renaissance. Modes are a whole different way of thinking about music and are extremely complex, and I won't get into them here. Unless you're planning to transcribe Gregorian or Roman chant into modern notation, you don't need to know about those yet.

What we call the 'circle of fifths' defines the 14 key signatures required to create major or minor scales starting on each of the notes on a piano keyboard.

Note that key signatures and keys are not the same thing. For any key signature--that is to say, two sharps or three flats or whatever--there are two actual keys in which the music can be written: the major key, and the relative minor key. For instance, the major scale which requires one sharp in the key signature starts on G; the minor scale which requires one sharp in the key signature starts on E.

Rule: The relative minor scale of any major key always starts on the note one and one half steps below the tonic (the first note of a scale) of the major scale.

Unfortunately, it's not quite that simple. There are three different types of minor scales: Natural, harmonic, and melodic. And that's where a lot of accidentals come into play....

The natural minor scale is defined entirely by the key signature. IOW, all the notes in that scale will be played without accidentals. For A minor (no sharps or flats; the relative minor of C major), the notes of the natural minor scale are thus: C, D, E, F, G, A, B, (C). For E minor (1 sharp), the notes of the natural minor scale are: E, F#, G, A, B, C, D, (E). The natural minor scale is the same both ascending and descending.

The harmonic minor scale contains a raised seventh, which is to say, the seventh degree of the scale is raised by a half-step so that it 'leads' more strongly to the tonic note of the scale. So, in A minor, the harmonic scale would be: A, B, C, D, E, F, G#, (A). The harmonic minor scale is played the same way both ascending and descending, just like the natural minor scale.

Now we come to the melodic minor scale, and things get a bit more interesting. The melodic minor raises both the sixth and seventh degrees of the scale by a half step each, but only on the way up the scale (ascending). When coming down the scale, those notes revert to their 'natural' value. So: for A minor, the ascending melodic minor would be A, B, C, D, E, F#, G#, (A), and the descending scale would be (A), G, F, E, D, C, B, A. I won't go into the historical reasons for the development of this scale--you can look that up on your own if you're interested--but the melodic minor is the scale most often used in much of Baroque and later music.

The reason I went into such detail about the minor keys is that most accidentals in music occur when working in those keys. (In popular music, the other common reason a note might be altered by an accidental is when using a 'small seventh' chord to modulate.) The important thing to understand about all this in relation to why some notes are 'spelled' one way sometimes and another way other times, is that a scale always has seven degrees, and they are always 'spelled' (notated) in scale order. In other words, if you're working in d minor, the seventh degree of the ascending melodic minor scale is NOT a D-flat, it is a C-sharp. The note 'C' is the seventh degree of any scale starting on D, thus, in a harmonic or ascending melodic minor scale, it must be raised by a half-step to C-sharp. If you lower the octave/tonic instead, you are making a musical statement about the key itself, and unless you know exactly what you are doing, you will be wrong 99% of the time. (As they say on television, 'Kids! Don't try this at home!')

The other thing to keep in mind is that triads--or basic three-note chords--are composed of a 'root' note, above which are placed a 'third' and a 'fifth' (I'm not going to discuss inversions here; look them up on your own). The third is two note names above the root; the fifth is four note names above the root. You must always use the proper note-names ('spellings') of the parts of the chord when you notate them; otherwise you will be telling the musicians something different. Example: If you are in c minor, and were to notate a chord as C/E-flat/G, you would be telling the musicians to play a tonic or "I" chord in root position. OTOH, if you notated that same chord as C/D#/G, you would be telling the musicians to play an augmented dominant or "V" chord in second inversion with an augmented third. Which is distinctly odd, not to say unheard of. Yes, it sounds the same, but it is not the same thing from a functional standpoint. Stuff like this matters, even if it makes your head ache at first.

MuseScore, as a program, attempts to 'guess' the proper spelling of notes with accidentals, but because the composer's intention in regard to any particular altered note is subject to interpretation, it cannot always guess correctly. For this reason, the program provides two ways for the user to correct its guesses. If you use the global command 'respell pitches', there is the possibility that a 'correct' spelling somewhere in the score will be changed to an 'incorrect' one through some fluke in the algorithm. If you use the J shortcut on a particular note, you can see the result immediately, and control what winds up in your final score.