Alleen and maar in their use as focus particles express some kind of restriction or limitation. They can also be combined:
In the first sentence alleen is used to exclude all other siblings. Maar in the second sentence expresses the same, but could imply that this is regrettable. This third example with alleen maar also carries that meaning.
Note that before numerals (as in the second example above) only maar can be used.
Ook and zelfs express the opposite of maar and alleen:
The words al, nog and pas focus on temporal elements: