ご質問どうもありがとうございました。
下記の言い方ではいかがでしょうか。
That person is really confident.
あの日は本当に自信を持ってる。
「That person」あの人
「really」[本当に](https://eikaiwa.dmm.com/uknow/questions/36491/)
「confident」[自信](https://eikaiwa.dmm.com/uknow/questions/36825/)持ってる。
They seem like they have a lot of confidence.
あの人はとても自信があるように感じる。
「seem like」な感じ
「have a lot of confidence」自信がある
confident が「自信がある」、confidence が「自身」という意味の英語表現です。
ご参考にしていただければ幸いです。
She is very confident in the way she walks and holds herself.
I feel pretty confident in the way I hold myself.
She looks pretty confident.
She is very confident in the way she walks and holds herself.
I feel pretty confident in the way I hold myself.
She looks pretty confident.
She is pretty sure of herself.
She exudes confidence in all she does.
I wish I could look that confident.
I wish I could look that sure of myself.
She has an air of confidence about her.
I think she acts very sure of herself.
I think she looks very confident.
She is very confident in the way she walks and holds herself.
（彼女の歩き方・[振る舞い](https://eikaiwa.dmm.com/uknow/questions/15904/)は[自信](https://eikaiwa.dmm.com/uknow/questions/36825/)に満ちています）
I feel pretty confident in the way I hold myself.
（自分に自信を持って振る舞うことができています）
She looks pretty confident.
（彼女はかなり自信を持っているように見えます）
She is pretty sure of herself.
（彼女はかなり自信を持っています）
She exudes confidence in all she does.
（彼女の行動からは自信がにじみ出ています）
I wish I could look that confident.
（あんな風に自信を持って振る舞えたらいいのに）
I wish I could look that sure of myself.
（あんな風に自信を持って振る舞えたらいいのに）
She has an air of confidence about her.
（彼女は自信に満ちあふれています）
I think she acts very sure of herself.
（彼女はすごく自信を持って行動していると思う）
I think she looks very confident.
（彼女はすごく自信を持っているように見えます）
When someone is 'assure of themselves' it means that they have great self confidence.
'Pluck' can also be used to describe someone who is very confident, and can be used as follows. "That man has got a lot of pluck."
assure of oneself'とは、非常に自信があるという意味です。
'pluck'を使って、とても自信のある人を表すこともできます。例えば：
"That man has got a lot of pluck."
あの人はすごく勇気がある。
Someone possessing confidence is someone that is sure of themselves. This means that they know their own abilities and strengths. They do not feel intimidated by a certain situation because they know that they can rely on their own abilities, experience and"know-how" to be successful in whatever task is before them.
Someone who has confidence is brave, they know that what they have to do might be daunting but their own fortitude helps them persist in a task ahead. They do not have a defeatist attitude but they bravely forge ahead, seeing the end goal in site.
Being sure is having reliance on yourself and your abilities. A donkey walking on a narrow road on a cliff-side is said to be "sure-footed", this is because it is confident it can traverse the path ahead successfully.
The two sentences you see provided above are great ways to express to your listener that a particular person is confident. In the second sentence you will see the term sure of oneself. This means to be confident or self assured. This term is commonly used in our everyday conversation, it would make a great addition to your vocabulary.
上記二つの例文は、自信を持った人を表す素晴らしい言い方です。
二つ目の例文には、"sure of oneself" という表現が使われています。これは「自信を持っている」という意味です。この表現は日常会話でよく使われます。ぜひ語彙に加えておいてください。
「自信がある」は英語で「have confidence」と「confident」で表現します。
だから、「自信がある人」は英語で「a person who has confidence」と「a confident person」で表現できます。
例文：
「彼は自信がある」
→「He has confidence」
→「He is confident」
「あんまり自信がない」
→「I don’t really have any confidence」
→「I’m not really confident」
ご参考になれば幸いです。
Someone who is very confident in their demeanour may also be described as being self-assured, self-possesed or self confident.
"John is a very self-assured young man and takes control of difficult situations quickly."
very confident'（自信に満ちた）な人のことは 'self-assured' 'self-possessed' あるいは 'self confident' と表すこともできます。
"John is a very self-assured young man and takes control of difficult situations quickly."
（ジョンは若いですが全く物おじせず、難しい状況にもすぐに対処できます）
Below I have included some examples used in everyday conversations.
Example: That young man is very confident because he makes public speeches all the time.
Example: He proved to me that he was very brave by singing in the karaoke competition for the first time.
Example: He experienced a difficult time when his singing audition went bad but he should a lot of resilience and did much better second time around.
以下に日常会話での使用例をお示ししました。
例：
That young man is very confident because he makes public speeches all the time.
（あの青年はいつも演説をしているので、自信に満ちあふれています）
例：
He proved to me that he was very brave by singing in the karaoke competition for the first time.
（彼は初めてカラオケコンテストで歌を披露してすごく勇気があることを証明しました）
例：
He experienced a difficult time when his singing audition went bad but he showed a lot of resilience and did much better second time around.
（歌のオーディションがうまく行かなかったとき、彼はつらい経験をしましたが、そこから見事に立ち直って2回目の挑戦では見違えるように良くなりました）
When you look like you are sure of something or know what you are talking about then you are said to be 'confident'
Sometimes the way you walk or dress makes you look more confident to other people
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