代表着已经做过或者说发生了的事情。 “了”

"了“ is strongly emphasized the former state 你昨天买什么了？我昨天买苹果了。is equal to 你昨天买了什么？我昨天买了苹果。Don't worry about it.

it just like an auxiliary word. make a sentences more lively . Ex:太好"了"( so good ) 買"了"什麼東西( what do you buy?) 太冷"了"( to cold)

As indicated in the answers by other users, 了 is indeed a very tricky word to use. There is two way to pronounce it. le and liao3. When used as an auxiliary, it's pronounced as liao3 and can be found in following circumstances. 1.It's used after a verb or an adjective to indicate the completion of an action, at a point in the past or before the beginning of another action. 2.It's used at the end of a sentence to indicate a change of situation or state. 3.It's used at the end or in the middle of a sentence to indicate a persuasion or request. 4.It's used at the end or in the middle of a sentence to express an exclamation. The 了 in 太早了！太冷了！fall under the 4th case. It's used here to express an exclamation. "It's so early! It's so cold!" This is easy to tell。 The 了 in “你昨天买什么了？我昨天买苹果了。” belong to the 2nd case as the 了 is at the end of the sentence。The 了 in 你昨天买了什么？我昨天买了苹果。“ belong to the 1st case as the 了 is right after the verb。 The 了‘s position is interchangeable in this particular case because the action described here in the sentence "Bought apple" can be interpreted either as a complete action or a change of situation。 There are cases when 了‘s position is fixed, for example: 打了一架（had a fight) In this case, the fight is a completed action which doesn't indicate a change of state. So 打一架了 doesn't really work。 水变臭了（the water became stink）In this case, becoming stink is a change of state and not even an action. So 水变了臭 doesn't really work。 I hope I have clearly explained the issue. If you still have doubt about it, please feel free to send me a message.

As a native Chinese. These are what I felt. Hope you can understand my English which I am still keeping learning. 太早了！太冷了！is more or less like a feeling from your mind " it's too early! it's too cold". 太早！太冷！could be felt like a sort comment like " too early! too cold" 你昨天买什么了？我昨天买苹果了。What have you bought yesterday? I have bought apples yesterday. 你昨天买什么？我昨天买苹果。What did you buy yesterday? I bought the apples yesterday.