Most keyboard shortcuts can be customized via the menu: select Edit→Preferences...→Shortcuts (Mac: MuseScore→Preferences...→Shortcuts). Below is a list of some of the initial shortcut settings.
Beginning of score: Home (Mac: Fn+←)
Last page of score: End (Mac: Fn+→)
Find (measure number, rehearsal mark, or pXX when XX is a page number): Ctrl+F (Mac: Cmd+F)
Next score: Ctrl+Tab
Previous score: Shift+Ctrl+Tab
Zoom in: Ctrl++ (doesn't work on some systems) (Mac: Cmd++); or Ctrl (Mac: Cmd) + scroll up
Zoom out: Ctrl+- (Mac: Cmd+-); or Ctrl (Mac: Cmd) + scroll down
Next page: Pg Dn; or Shift + scroll down (Mac: Fn+↓)
Previous page: Pg Up; or Shift + scroll up (Mac: Fn+↑)
Next measure: Ctrl+→ (Mac: Cmd+→)
Previous measure: Ctrl+← (Mac: Cmd+←)
Next note: →
Previous note: ←
Note below (within a chord or on lower staff): Alt+↓
Note above (within a chord or on higher staff): Alt+↑
Top note in chord: Ctrl+Alt+↑ (Ubuntu uses this shortcut for Workspaces instead)
Bottom note in chord: Ctrl+Alt+↓ (Ubuntu uses this shortcut for Workspaces instead)
Next element (accessibility): Alt+→
Previous element (accessibility): Alt+←
Begin note input mode: N
Leave note input mode: N or Esc
1 ... 9 selects a duration. See also Note input.
Half duration of previous note: Q
Double duration of previous note: W
Decrease duration by one dot: (as of version 2.1) Shift+Q (e.g. a dotted quarter note becomes a quarter note; a quarter note becomes a dotted eighth note)
Increase duration by one dot : (as of version 2.1) Shift+W (e.g. an eighth note becomes a dotted eighth note; a dotted eighth note becomes a quarter note)
To select a voice in note input mode.
Voice 1: Ctrl+Alt+1 (Mac: Cmd+Alt+1)
Voice 2: Ctrl+Alt+2 (Mac: Cmd+Alt+2)
Voice 3: Ctrl+Alt+3 (Mac: Cmd+Alt+3)
Voice 4: Ctrl+Alt+4 (Mac: Cmd+Alt+4)
Pitches can be entered by their letter name (A-G), or via MIDI keyboard. See Note input for full details.
Repeat previous note or chord: R (the repeat can be of a different note value by selecting duration beforehand)
Repeat selection: R (The selection will be repeated from the first note position after the end of the selection)
Raise pitch by octave: Ctrl+↑ (Mac: Cmd+↑)
Lower pitch by octave: Ctrl+↓ (Mac: Cmd+↓)
Raise pitch by semi-tone (prefer sharp): ↑
Lower pitch by semi-tone (prefer flat): ↓
Raise pitch diatonically: Alt+Shift+↑
Lower pitch diatonically: Alt+Shift+↓
Change enharmonic spelling in both written and concert pitch views: J
Change enharmonic spelling in current view only: Ctrl+J (Mac: Cmd+J)
Rest: 0 (zero)
Add interval above current note: Alt+[Number]
Flip direction (stem, slur, tie, tuplet bracket, etc.): X
Mirror note head: Shift+X
Increase stretch of measure(s): }
Decrease stretch of measure(s): {
Line break on selected barline: Return
Page break on selected barline: Ctrl+Return (Mac: Cmd+Return)
Staccato: Shift+S
Tenuto: Shift+N
Sforzato (accent): Shift+V
Marcato: Shift+O
Grace note (acciaccatura): /
Crescendo: <
Decrescendo: >
Staff text: Ctrl+T (Mac:Cmd+T)
System text: Ctrl+Shift+T (Mac: Cmd+Shift+T)
Tempo text: Alt+Shift+T
Rehearsal Mark: Ctrl+M (Mac: Cmd+M)
Enter lyrics on a note: Ctrl+L (Mac: Cmd+L)
Previous lyric syllable: Shift+Space
Next lyric syllable: if the current and the next syllables are separated by a '-': -, else Space
Move lyric syllable left by 0.1sp: ←
Move lyric syllable right by 0.1sp: →
Move lyric syllable left by 1sp: Ctrl+← (Mac: Cmd+←)
Move lyric syllable right by 1sp: Ctrl+→ (Mac: Cmd+→)
Move lyric syllable left by 0.01sp: Alt+←
Move lyric syllable right by 0.01sp: Alt+→
Up to previous stanza: Ctrl+↑ (Mac: Cmd+↑)
Down to next stanza: Ctrl+↓ (Mac: Cmd+↓)
For more lyric shortcuts, see Lyrics.
Timeline: F12 (Mac: Fn+F12)
Play Panel: F11 (Mac: Fn+F11)
Mixer: F10 (Mac: Fn+F10)
Palette: F9 (Mac: Fn+F9)
Inspector: F8 (Mac: Fn+F8)
Piano Keyboard: P
Selection filter: F6
Display full screen: Ctrl+U
Toggle visibility on selected element(s): V
Show Instruments dialog: I
Toggle multi-measure rests on or off: M
Reset element to default location: Ctrl+R (Mac: Cmd+R)