Thank you for your help !
I'm afraid that I've tried to write something too difficult for me... I wanted to say :
"Today, I had a lot of work, so I didn't have a lot of time for lunch" (for eating at lunch time). (I did eat, but quickly)
"My oldest son had a problem."
"He didn't know his password". (he forgot it)
"As for me, I knew his password".
(He phoned me yesterday because he needed the password for his exams registrations that we did together on last school break. Thankfully, I had it and I was able to give it to him...)
But writing such a complex diary entry was really beyond what I am able to understand once translated, so not useful for the beginner I am. I really don't know enough of korean language, and when I try to use Papago, I don't understand how my sentences are translated, even when I look carefully at each word. So it doesn't help me to progress.
I'll be on school break again this saturday for two weeks, so I will study more.

Thank you so much additional explanation! Let me write down in Korean what you wanted to say;
오늘 일이 많아서 점심을 먹을 시간이 없었어요.
큰아들에게 문제가 있었어요.
그는 비밀번호를 몰랐어요. (그는 비밀번호를 잊어먹었어요.)
저는 그가 찾고 있는 비밀번호를 알고 있었어요.

Yes, I understand that writing complex sentences in Korean is quite challenging. Let's make a sentence step by step :) I will help you! You are doing very well!

Thank you so much ! I would really be honored to help you in french as you do for me in korean.
You didn't change the korean word for "problem" (문제), but previously, you said that in my sentence, it looks like my son has a mental difficulty or some illness. So which korean would you use for "to have a problem" to fit my sentence better ?
I realise that in european languages, at least in english, italian, spanish and especially in french, we use so many prepositions. It might be hard for foreigners with very different languages, but for me, I have to say that I struggle a lot to understand how the korean sentence is built ! ^^ In the last sentence that you translated for me, I need to learn more to understand why you use the verb 있다 in addition to verbs (찾 and 알) that drive the meaning.
Right now, I am trying daily to read some texts in korean to help me understand how korean sentences work (I'm reading a webtoon and some "easy" articles).
I also wanted to tell you that my english is not top notch, so I probably make a lot of errors as I don't check what I write with a translator. Sorry for that. :) Thank you again.
Now I'll write my fixed text on my diary. I read my korean diary several times to help me to learn korean, that's why it's so important that what I wrote was right, and also that I can understand it.

I checked my grammar books and I am quite ashamed that I still didn't know about present tense... So now I understand what is 있다 in the sentence !

To avoid any misunderstanding, the most natural way to say it is '큰아들에게 일이 있었어요' (Something happened to my son). The word '문제' is perfectly fine, but the particle changes the nuance: '은' implies 'He IS a problem' (Identity), whereas '에게' implies 'He HAS a problem' (Situation). This is a very subtle and advanced distinction, so don't worry if it feels difficult right now!

(J'utilise un traducteur IA.) Je suis d'accord avec toi : les langues européennes utilisent beaucoup de prépositions. En revanche, le coréen utilise beaucoup de particules (Josa). Comme une préposition, une simple particule peut changer toute la nuance ! En tant que native, je les utilise instinctivement, donc c'est parfois difficile à expliquer, mais apprenons ensemble petit à petit.