"They are not interested in playing tennis."
Translation:A ellas no les interesa jugar al tenis.
I wrote "ellos no les interesa jugar tenis" - can anyone explain why i have to include the article "al" and the rule around this please??
Not sure, but I think ESTAN (estar) isn't a good form of "to be" in this sentence.
The way it was explained to me many years ago... Ser is used in basic inherent cases, things that are not likely to change. Estar is used when it could change. The difference between, "They are not interested in playing tennis" and "They are not interested in playing tennis right now". In the first example, they are not interested and that is not likely to change.
Interesar is a "reverse verb" (my term) like gustar. Playing tennis does not interest them. Interesar conjugates for the playing not for them. That is also why you need "a" ellos, not just ellos. Other "reverse" verbs include aburrir, fascinar, bastar, importar, caer bien (mal), dar asco, molestar, disgustar, paracer, doler, picar, encantar, quedar, faltar, volver loco. There may be others. You just have to memorize them.