YAKINDA=soon. YAKIN=near/close YAKININDA= close in comparison to other place YANINDA=next to smth
ex. Yakında sinemalarda=soon in the sinema.
Evim çok yakın=my house is very close/near.
Okul hamamının yakıninda=the school is close to the hamam.
Evimin yaninda bir mağaza var=there is a shop next to my house
All in all the proposed sentece by duolinguo is wrong.dont take it into account.
In the second sentence, you're using the locative suffix -DA (-TA). As Ektoraksan mentioned above, "yakında" is like saying "at a close distance" and has the same meaning as "yakın" ("near" or "close").
To say "The lake was close," you would need to add the tense suffix -DI and the personal suffix. (In this case, there isn't one because "lake" is third person singular.) "Göl çok yakındı" or "Göl çok yakındadı."