Grammar Info
N4 Lesson 10: 14/18

why don't you...?, what if you did...?

It can be further contracted to たら by itself in very casual conversations.

Verb[たら]+ どう + (だ)?
Verb[たら]+ どう + ()?
Verb[たら]+ どう + です + か
Register Standard
使用域 一般
When the 接続助詞(せつぞくじょし) (conjunction particle) たら is used before the 副詞(ふくし) (adverb) どう, it primarily presents a situation, then asks a question about it potentially happening. This construction can be translated as ‘why don’t you (A)?’, or ‘what if you did (A)?’. Realistically, it just means ‘when (A), how about that?’. It is often used for giving advice.
To use this expression, convert the verb that you want to use into its たら form, and then add どう.
  • 食(た)べ量(りょう)減(へ)らしたらどう
    Why don't you reduce the amount you eat?
  • テレビつかないの?叩(たた)いてみたらどう
    Your TV doesn't turn on? Why don't you try hitting it?
Sometimes たら is used by itself, with a tone that implies that it is a question (or simply by adding a question mark in written language). This is an abbreviation of たらどう, and carries the same nuance.
  • 暇(ひま)なら外(そと)行(い)ったら
    If you are bored, why don't you go outside?
or may be added to the end of たらどう, to emphasize the ‘question’ even further. However, they are not required.
  • 警察(けいさつ)電話(でんわ)してみたらどう
    Why don't you try calling the police?
  • 苦(にが)なら砂糖(さとう)入(い)れたらどう
    If it is bitter, why don't you add some sugar?
Despite being used for giving advice, たらどう may be interpreted as criticism. Due to this, care will need to be taken with your tone of voice.
  • たま頭(あたま)使(つか)ったらどう
    Why don't you use your head occasionally? (You should try thinking for once)
Slow Male
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Why don't you go to play in the park?
Why don't you go pick up that garbage over there?
Why don't you try dieting?
[ダイエット means "to lose weight by any method" and is not tied to just diet (i.e. it can include exercise)]
Genki II 1st Edition
Page 36
Genki II 2nd Edition
Page 58
[DBJG] A Dictionary of Basic Japanese Grammar
Page 457
Tae Kim's Japanese Grammar Guide
Page 152