When a verb comes after pleje, at is always added to the infinitive (like in English "to tend to do something" the "to" is added after "tend" in the same way)
This is the case for all verbs used in this way except modal verbs (kan, vil, skal, må, bør etc.)
Thankyou and also I saw this in another sentence, but what is the difference between "Prøver" and "forsøger" in the meaning for " Jeg prøver. eller Jeg forsøger"? is there a reason to use one or the other??
The difference is small. The main meanings of the two words are almost exactly the same. "Prøve" is "try" and "forsøge" is "attempt". The difference is similar to the difference between those two English words. "Forsøge" implies that you want to achieve something specific. "Prøve" can be used both when you are trying to achieve something and when you're just trying something. "Forsøge" sounds a little bit more formal. It suggests a slightly more complicated effort than "prøve", although "prøve" may also be used about complicated efforts.