I suffer from bad hayfever and I usually take pseudophedrine tablets. If I want to drink alcohol what will happen?

Probably not much. If you are healthy, without any significant blood pressure or heart issues and you drink in moderation you should be fine.
Nothing. Pseudoephedrine is a stimulant in addition to its decongestant properties. Alcohol is a depressant. The stimulant and depressant properties will likely cancel each other. The decongestant properties of pe will remain and the central nervous effects of alcohol will remain. Be cautious in combining... You may be more alert but you can still get just as intoxicated.