Below is a list of the most basic shortcuts that will get you started. These shortcuts are for the Across Online Client and Across Translator Edition - some shortcuts may differ in crossWeb. You can usually right-click a segment to see all the actions you can take and the shortcuts for them.
CTRL + ALT + ENTER to confirm a segment to their final paragraph state, update the TM, and move to the next segment
CTRL + SHIFT + number to copy a tag from source to target (1, 2, 3 etc. depending on the number of tags in the source)
CTRL + ALT + number to copy placeables with a blue overline from source to target (1, 2, 3 etc. depending on the number of placeables in the source)
CTRL + number of term entry (hold down both keys) to insert crossTerm entry
ALT + INSERT to insert the highest crossTank TM match into target
ALT + PLUS to move to the next segment
ALT + MINUS to move to the previous segment
CTRL + SHIFT + C to search Concordance on highlighted term in source or target
ALT + Page Down to copy source to target
For a full list of all shortcuts for the crossDesk editor view, see the this page in the Across online help.