怎么说 "I didn't know you could speak chinese"
I'm not sure how to say "I didn't know" (PAST TENSE) in mandarin.

Maybe I'm confusing myself, sometimes past tense sounds too easy and strange in my head.
I've tried coming up with sentences myself, but they don't sound right:

我不知道你会说中文

Question

怎么说 "I didn't know you could speak chinese"
I'm not sure how to say "I didn't know" (PAST TENSE) in mandarin.

Maybe I'm confusing myself, sometimes past tense sounds too easy and strange in my head.
I've tried coming up with sentences myself, but they don't sound right:

我不知道你会说中文 <--- No indication that it is past tense (for as far as I know)
我不知道你能说中文

Answer

There isn't any grammar in Mandarin for past tense. 
For example. 
I am eating apple. 我正在吃蘋果 
I ate an apple. （我吃了一個蘋果- already finished but without time, you can only know it is already past)
I ate an apple yesterday. (我昨天吃一個蘋果）

About your question, for me, I will say 我不知道你會說中文
but for Chinese speaker who from China, I think they also say 我不知道你能說中文 as well.

Answer

chinese do have past tense!
知道 is an exception. we can always say 不知道。
I didn't know。我（以前）不知道。
Similar example: I was a teacher.  (我以前是一个老师。)

我觉得下面的这个回答有点问题：
I ate an apple yesterday. (我昨天吃一個蘋果）
我觉得“我昨天吃了一个苹果”比较好。

Answer

我不知道你会说中文 means the same with 我不知道你能说中文, but 我不知道你会说中文 is more used and natural. If you want to use “能” you can say " 我不知道你还能说中文啊" (surprised).

Answer

我不知道你能说中文

Answer

"我不知道你会说中文” is better and sounds more natural to me~