Thankful.
I will try to answer these questions very briefly and usefully and tell you why the answers you gave are wrong or incomplete at best.
The explanations I give are for the general situation and I will not deal with the exceptions.
1. Entity or Subjekt or subject:
The word or part of the sentence that the rest of the sentence is about and we want to talk about it.
All sentences, if they are grammatically complete and correct, must have an entity. That is, the existence of the institution is not limited to news sentences. Now I will give some examples.
Jack has one brother.
In this sentence there is Jack and the rest of the sentence is about Jack.
Heute is the last day of the month.
In this sentence Heute is the entity and the rest of the sentence is about Heute.
But the entity is not always a word or a person. For example
The first President of the USA was a man.
Here is the institution Der erste Präsident der USA and the rest of the sentence is about this phrase.
Or, for example, the sentence can be a question:
Wann bist du yesterday Abendgekommen?
Here is du entity.
2. Doer (I don’t know German)
As you said, the subject is the person or thing that does the action of the sentence. For example:
Er spielt sehr gut Klavier.
Er is the subject here.
3. What is the difference between these two?
In fact, this is the most important question and the answer to it determines many things.
The first difference is that all sentences have a subject, but not all sentences have a subject. In fact, they only have active sentences in which action has taken place. For example, in the following sentence
Heute is the last day of the month.
There is no subject because there is no action. But this sentence has an institution.
The difference between them is that the entity is always the focus of the sentence, but the subject is not necessarily the focus of the sentence. This means that the institution and the subject are not necessarily the same. For example, in the following sentence
Jack spielt sehr gut Klavier.
It is called Jack because the rest of the sentence is about him. But does it have an active sentence? Yes, because there is an action in the sentence and that is playing the piano. But who is the subject of the sentence? Jack again. Because Jack is doing the action of the sentence. So here, Jack is both an entity and an agent, but for two different reasons!
But in the following sentence:
This room is used by the students.
The entity Dieser Raum is there because it is the focus of the sentence and we are saying something about it. Does it have an active sentence? Yes, because it contains an action. But who is the subject here? The answer is Studenten. In fact, here we have a sentence that has both subject and subject, but they are not the same.
The last point is that all the sentences that have verbs do not necessarily have a subject, but they still have a subject. For example
Die Sitzung wurde verschoben.
Here is the entity Die Sitzung, but there is no subject.
If you understand what I’m saying, we can easily go to the concept of passiv and aktiv. Ask beforehand if you have any questions.
This post is written by EhSep