*Can I say this casually?
This mean you would like to know if you can use the phrase in a casual way.
*Is this expression informal?
If n expression is informal it mean you can use it among your friends or family but not in a business setting.
*Can I say this casually?
これをカジュアルに言えますか？
このフレーズをカジュアルに言えるかどうか知りたいということです。
例文
*Is this expression informal?
この表現は口語ですか？
informal（口語の）な表現は、友達や家族の間では使えますが、ビジネスの場面では使えません。
When learning to speak conversational English, it is always a good idea to know which expressions are formal and informal.
Informal expressions generally are colloquial and casual.
To ask if an expression can be used informally ask:
"Is this an informal expression?"
or
"Is this an everyday expression?"
or
"Is this expression used casually?"
If the teacher is a native speaker or an advanced English speaker, they should be able to tell you which expression is common and which is bookish.
例：Is this an everyday expression?
「これはいつも使う表現ですか？」
例：Is this an informal expression?
「これはカジュアルな表現ですか？」
例：Is this expression used casually?
「この表現はカジュアルな場面で使われますか？」
英語の会話表現を学んでいるときは、フォーマルな表現とカジュアルな表現に気をつけると良いでしょう。
ネイティブスピーカーの先生や、英語力の高い先生は口語表現と文語表現もわかっているはずです。
During an online English lesson, you would like to ask your teacher if a certain phrase is a casual expression.
In that case, you can try one of the above-suggested example sentences which may be appropriate for any given situation. There are always a variety of ways to respond to someone or make a comment - depending on the context and other factors such as nationality, appropriacy and body language.
Can I say this expression in a business context or a casual situation?
In English grammar, there are certain expressions that can only be said to friends or other social situations, and, there are other expressions that are appropriate for business situations. We call expressions that are appropriate for social interactions, 'informal', and those that are appropriate for business situations, 'formal'. If you say that a certain expression if casual, it means that it is 'informal'.
So, you may ask your teacher:
Is this a formal or informal expression?
or
Can I say this expression in a business context or a casual situation?
英語では友達に対してしか使わない表現と、ビジネスシーンで使っても問題の無い丁寧な表現があります。友達やカジュアルな場で使える表現は 'informal'、ビジネスシーンなどで使う丁寧なものは 'formal' と呼びます。もしこの表現は ”casual” だと言われた場合は、'informal'を意味します。
なので、先生にはこのように質問するといいでしょう。
Is this a formal or informal expression?
（これはフォーマルな表現ですか、それともインフォーマルな表現ですか）
Can I say this expression in a business context or a casual situation?
（この表現はビジネスの内容で使えますか？それともカジュアルなシチュエーションですか？）
These are three different ways that we can ask if a phrase is used normally in everyday life. The most common word would be, "casual expression," however you can also use the word, "colloquial," to describe an expression that is used in a certain country/region/town on an everyday basis. Lastly, we can ask about how, "often," something is used in everyday speech.
どれも同様の意味です。
・Is this a casual phrase? （これはカジュアルな言い方ですか）
phrase=句、成句、熟語、慣用句、決まり文句
・Is this expression informal? （これはくだけた表現ですか？）
expression=表現(すること)、表われ、しるし、(言葉の)言い回し
・Is this a casual expression? （これはカジュアルな表現ですか？）
少しでも参考になれば幸いです。
'Is this an informal expression?' is a question you would ask your teacher if you would like to confirm if the phrase or expression you are using is casual/informal or formal.
アメリカ合衆国 
日本 
南アフリカ 
ニュージーランド 
イギリス 
