曾经 ceng2 jing1........过guo4 eg. 我曾经有过一条狗。（it means i once had a dog, but i dont have at the moment. eg2.我曾经在那里买过东西。（it means I once bought some stuffs there, but I dont go to buy at the moment.) These are just similar meaning. The accurate expression should be 曾经...过，但现在没有....。 我曾经有过一条狗，但现在没有养。 or 我曾经在那里买过东西，但现在没有在那里买。

你可以说“（过去、以前）曾经..."。 比如，我曾经养有一只狗。

I used to own a dog = 以前 我 有 一 只 狗 了

Written in classical Chinese : I (吾) used to (嘗) own (豢) a (一) dog (犬).

1. "ceng2 jing1" is the word you could use before the verb and after the subject, plus using "guo4" after the verb. Like "I used to own a dog"-----"wo3 ceng2 jing1 yang3 guo4 yi4 zhi1 gou3". 2. And if you wanna emphasize more on the past tense, you could use "yi3 qian2" in front of the whole sentence, plus using "guo4" after the verb. E.g. "I used to own a dog"----"yi3 qian2 wo2 yang3 guo4 yi4 zhi1 gou3". Either way is ok. Plz notice that "guo4" is the most common way for us to express the action happened in the past. Good luck to your Chinese learning! :)