You use hence in expressions such as `several years hence' or `six months hence' to refer to a time in the future, especially a long time in the future. The gases that may be warming the planet will have their main effect many years hence
therefore: (used to introduce a logical conclusion) from that fact or reason or as a result; "therefore X must be true"; "the eggs were fresh and hence satisfactory"; "we were young and thence optimistic"; "it is late and thus we must go"; "the witness is biased and so cannot be trusted"
(used to introduce a logical conclusion) from that fact or reason or as a result; "therefore X must be true"; "the eggs were fresh and hence satisfactory"; "we were young and thence optimistic"; "it is late and thus we must go"; "the witness is biased and so cannot be trusted"
You use hence to indicate that the statement you are about to make is a consequence of what you have just said. The trade imbalance is likely to rise again in 1990. Hence a new set of policy actions will be required soon = therefore, thus