I want the Conservatives out (if you hadn't guessed from my Facebook posts ) but I don't know whether it's safe to vote Labour in my constituency - I don't really understand the principal of tactical voting.
In 2015 the votes were:
Plaid 15,140 Labour 9541 Conservatives 8336
My husband votes Plaid, but I want to vote Labour if it's safe. Not least because one of my best friends is Labour's parliamentary candidate for the next constituency and I want to be able to look her in the eye! However I'm not averse to voting Plaid if it does the job.
But you need to make the best guess you can as to whether there's a chance of the Conservatives getting your constituency - if enough people don't bother to vote, say, or some Plaid Cymru voters switch to Conservative (is that even possible . If there's little risk you should vote for what you want not against what you don't want.
If, like most people, you live in a "safe seat" that is an almost certain win for one party, then your vote doesn't really matter and you might as well vote for whomever you support.
If you live in a marginal seat, and support one of the parties that stands a chance of winning it, then vote for the party you support.
If you live in a marginal seat, and the party you support doesn't stand much chance of winning it, then decide which of the parties that might win you least dislike and vote for that party.
One problem with tactical voting is that if most people did it, each constituency would become just a two-way fight with no third, fourth, fifth, ... parties ever able to gain a foothold.
Another problem with tactical voting is that you have to rely either on past election results or opinion polls for information about which parties stand a chance of winning in your constituency - and lately things have been so volatile that neither past results nor opinion polls are a reliable guide.
There's a lot to be said for just voting for the candidate or party you most like or least dislike - that keeps things simple!