I'm so tired that I can't think straight＝疲れすぎてまともに考えられない
I'm so tired = とても疲れている
think straight＝[普段通り](https://eikaiwa.dmm.com/uknow/questions/4404/)考える
「[疲れている](https://eikaiwa.dmm.com/uknow/questions/87687/)」事を強調する場合は「I'm so tired」を先に言い、「まともに考えられない」事を強調したい場合は逆に言って
I can't think straight because I'm so tired
疲れすぎてまともに考えられない
と言いましょう。
In American English, it is acceptable to say that you cannot 'think correctly' to mean that you can't think well. In the beginning of the sentence, you have explained why you cannot think well, which is because you are tired. Your teacher should be able to understand you without a problem.
I hope this helps. :)
Usually it's enough to explain that you 'can't concentrate' on your work. There may be a wide variety of reasons why, but it may not be appropriate to explain any of them - anyway it is not really necessary.
"I am so tired that I cannot think clearly."
Tired, meaning you need to sleep or rest.
Think clearly, because you are tired you are not alert and things do not make sense.
"I am exhausted and I cannot think straight."
Exhausted, Meaning you are tired and without energy to do anything.
Think straight, this has the same idea as think clearly.
I am so tired that I cannot think clearly.
私は、とても疲れていてよく考えられません。
tired－睡眠、または休憩が必要という意味
cannot think clearly－疲れてるため、注意を払ってなく理解してない
I am exhausted and I cannot think straight.
私は、疲れ切っていてまともに考えられません。
exhausted－何もする気力がなく疲れ果てているという意味
think straight－think clearlyと似た意味
I've had a long day - This implies that you have already been working that day for many hours and are tired.
My brain just isn't switched on this evening - This is an informal way of saying that you are finding it hard to concentrate due to being tired.
I am worn out - This is a alternative to saying you are tired. It could mean physically or mentally tired. Usually in English it relates to both mental and physical fatigue.
I've had a long day－その日すでに長時間仕事をしていて疲れてるという意味を含んでます。
My brain just isn't switched on this evening
（今夜は、私の頭のスイッチが入りません）
上記は、疲れてるため集中するのが困難だと言うための日常会話で使う表現です。
I am worn out－疲れたと言うためのもう一つの表現です。肉体的、または精神的にくたくたに疲れてることを意味します。通常、英語では精神的、肉体的の両方の疲労に関連します。
When saying you are tired after a long day you could use the term 'I've had a long day' or 'I'm tired' they would describe the same thing.
If you are tired then you sometimes can't think straight, we also used the term 'concentrate'
The simple term would be 'to think correctly'
長い一日の後、「疲れた」と言いたいときは、以下のフレーズが使えます。
'I've had a long day'（長い一日でした）
'I'm tired'（疲れました）
どちらも同じ意味になります。
疲れているときは、頭が働かない（can't think straight）ことがあります。'concentrate'（集中する）という単語も使われます
シンプルな言い方は'think correctly'（まともに頭が働く）でしょう。
I'm worn out is another way of saying that you're tired.
Exhausted means very tired.
It's been a long day is understood to mean it's been a tiring day.
Concentrate means to give something your full attention (in this context)
Not thinking clearly or not thinking straight mean not concentrating fully, generally because you are tired or there is something else on your mind
'I'm sorry teacher that I'm not giving you my full attention. I'm very tired after a hectic day at work and just can't concentrate. Shall we try again tomorrow?'
I'm worn out'は'I'm tired'（疲れた）の別の言い方です。
'exhausted'は「とても疲れた（very tired）」という意味です。
'It's been a long day'は「大変な（骨の折れる）1日だった」という意味になります。
'concentrate'は（この文脈では）「注意を集中する」という意味です。
'not thinking clearly'又は'not thinking straight'は、疲れていたり気になることがあって集中できないことを表します。
例文
'I'm sorry teacher that I'm not giving you my full attention. I'm very tired after a hectic day at work and just can't concentrate. Shall we try again tomorrow?'
（先生、集中力を欠いてしまってすみません。仕事でてんてこ舞いして、とても疲れたので集中できません。明日またやりましょうか？）
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