Why are these sentences with conditional forms not correct?

Question

I'm struggling a lot with the use of conditional forms, especially using ～たら and ～なら. I recently had the following sentences corrected by my Japanese teacher:

a. 専門書を買ったら、私にも見せてください。

b. 専門書を買うなら、私にも見せてください。

c. 今年の夏、国へ帰ったら、私も連れて行ってください。

d. 今年の夏、国へ帰るなら、私も連れて行ってください。

e. 料理を作ったら、私も手伝いましょう。

f. 料理を作るなら、私も手伝いましょう。

I thought in these sentences both conditional forms were okay, but only (b), (c) and (f) were marked as correct by my teacher. Why is that?

Answer 1

(a), (d) and (f) are correct. Compare:

「V1たらV2する」- V2 occurs after V1. "will do V2 when/after V1 has completed"
「V1ならV2する」- V2 occurs at the same time as, or before V1. "will do V2 if you (are going to) do V1"

◎ a. 専門書を買ったら、私にも見せてください。
× b. 専門書を買うなら、私にも見せてください。

(a) 買ったら is correct because the action 見せる will occur after the action 買う has completed. (b) sounds strange because it would sound like "If you're going to buy the book, please show it to me before you do so."

◎ d. 今年の夏、国へ帰るなら、私も連れて行ってください。
◎ f. 料理を作るなら、私も手伝いましょう。
× c. 今年の夏、国へ帰ったら、私も連れて行ってください。
× e. 料理を作ったら、私も手伝いましょう。

(d) and (f) are correct because 連れて行く/手伝う will occur at the same time as 帰る/作る, NOT after the action 帰る/作る has completed. (c) and (e) sound strange because these would sound like "Please take me with you when you've returned home" "I'll help you when you've cooked".

A few examples:

「飲んだら乗るな、乗るなら飲むな。」 "Don't drink and drive."
「捨てるなら、もらわない」 "If you can't reuse it, refuse it"
「帰ったら手を洗おう。」 "Wash your hands when you get home."