Получатель быстро ответил, что видит только первый слайд, а диаграмма на втором не открывается.

Получатель means recipient or the person who received something.

In this sentence, it most likely means the person who received the email, file, or presentation.

It is a normal word, especially in:

• formal language
• business or technical contexts
• interfaces and forms

In everyday speech, Russian speakers might sometimes use a different expression depending on context, but получатель is perfectly natural here.

Ответил is the perfective past form of ответить. It shows a completed single action: the recipient gave a reply.

That fits this sentence well, because the focus is on the fact that he replied.

If you used отвечал instead, it would sound more like:

• he was in the process of replying
• he replied repeatedly
• the process of replying is being emphasized rather than the completed result

So быстро ответил = replied quickly is the natural choice.

Что introduces the content of what the person said. Here it works like English that:

• ответил, что... = replied that...

Russian uses что very commonly after verbs of saying, thinking, reporting, noticing, and so on.

Unlike English, where that is often omitted, Russian usually keeps что in sentences like this.

This is very normal in Russian.

Russian does not always shift tenses backward the way English often does in reported speech. After a past reporting verb, the second verb can stay in the present if the situation is current or simultaneous from the speaker's point of view.

So:

• ответил, что видит... = he replied that he sees / can see...

This does not sound wrong or inconsistent in Russian.

If the seeing had happened earlier and was clearly in the past, then a past form might be used instead.

The verb видеть takes a direct object, so слайд is in the accusative case.

However, for many inanimate masculine singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: первый слайд
• accusative: первый слайд

That is why the form does not visibly change here.

The adjective первый also matches the masculine inanimate accusative, which in this pattern also looks like the nominative.

This is an example of omission of a repeated noun.

The full form would be:

• на втором слайде = on the second slide

But because слайд was already mentioned, Russian can omit it when the meaning is obvious:

• на втором = on the second one

Grammatically, втором is in the prepositional case, agreeing with the understood noun слайде.

So the sentence really means:

• диаграмма на втором (слайде)

А often connects two ideas that are being contrasted or set side by side.

Here the contrast is something like:

• he can see only the first slide,
• whereas / but the diagram on the second one does not open.

So а works very naturally.

Rough comparison:

• и = and, simple addition
• но = but, stronger contradiction
• а = and/but/whereas, often a softer contrast or comparison

In this sentence, а is the most natural connector.

Because диаграмма is the subject of не открывается.

The structure is:

• диаграмма ... не открывается
• the diagram ... does not open

In Russian, the subject is normally in the nominative case, so диаграмма stays nominative.

Открывается comes from открываться, which is the imperfective reflexive/intransitive partner of открывать.

Compare:

• я открываю файл = I open the file
• файл открывается = the file opens / is opening
• файл не открывается = the file doesn’t open / won’t open / isn’t opening

With software, files, pages, links, slides, and diagrams, Russian very often uses this kind of reflexive verb to mean that something fails to open.

So диаграмма ... не открывается is a very natural way to say:

• the diagram does not open
• the diagram won't open
• the diagram isn't opening

The exact English translation depends on context.

These forms give different time meanings:

• не открывается = doesn't open / isn't opening
  This describes the current problem or general situation.

• не открылась = didn't open
  This refers to a specific attempt in the past.

• не откроется = won't open
  This points to the future or expresses expectation that it will fail.

In your sentence, the speaker is describing the problem as it exists at the moment, so не открывается is the best fit.

There are two commas for two different reasons.

1. After ответил
   Russian puts a comma before что when it introduces a subordinate clause:

• ответил, что видит...

2. Before а
   А joins two clauses, and Russian normally separates such clauses with a comma:

• ...видит только первый слайд, а диаграмма... не открывается

So the commas are standard Russian punctuation, not optional decoration.

Russian word order is more flexible than English, but the original order is neutral and natural:

• Получатель быстро ответил, что...

This gives a straightforward flow:

1. who did it
2. how
3. what he said

Other orders are possible, but they change the emphasis. For example:

• Быстро ответил получатель...
  This emphasizes quickly replied

• Получатель ответил быстро...
  This is also possible, with a slightly different rhythm

So the sentence as written sounds like a normal, unmarked way to say it.