Mein Browser öffnet heute zu viele Seiten gleichzeitig.

Because Browser is singular and masculine: der Browser.

The possessive mein changes its ending depending on gender, case, and number, a bit like ein does:

• mein Browser = my browser
• meine Browser = my browsers
• meinen Browser = my browser, when it is masculine accusative

In this sentence, Browser is the subject, so it is in the nominative singular masculine form: mein Browser.

In German, all nouns are capitalized, not just proper names. So Browser gets a capital B because it is a noun.

Its gender is masculine: der Browser.

Even though Browser comes from English, it is treated like a normal German noun. It is best to learn it together with its article: der Browser.

The infinitive is öffnen, meaning to open.

The subject is mein Browser, which is third person singular: he / she / it.
So the verb takes the third person singular form:

• ich öffne
• du öffnest
• er / sie / es öffnet

So mein Browser öffnet means my browser opens / is opening.

German usually uses the simple present for both:

• opens
• is opening

So Mein Browser öffnet ... can mean either, depending on context.

If you really want to stress that it is happening right now, German often adds a word like gerade:

• Mein Browser öffnet gerade zu viele Seiten gleichzeitig.

But in many everyday sentences, the plain present tense is enough.

Here, zu means too.

So:

• viele Seiten = many pages
• zu viele Seiten = too many pages

This is not the same zu used before an infinitive, as in zu machen.
Here it is simply describing the quantity as excessive.

Because viele can work by itself, much like English many.

So:

• viele Seiten = many pages
• zu viele Seiten = too many pages

You do not need an article here.

If you added one, the meaning would become more specific:

• die vielen Seiten = the many pages

In your sentence, the pages are not being identified as a specific set, so zu viele Seiten is the natural choice.

Seiten is in the accusative plural because it is the direct object of öffnet.

The browser is doing the opening, and the pages are what it opens.

Singular and plural are:

• die Seite = the page
• die Seiten = the pages

In German, the nominative plural and accusative plural often look the same, and that is true here too: Seiten.

Because German main clauses normally follow the verb-second rule.

In this sentence:

• Mein Browser = first position
• öffnet = second position

After that comes the rest of the sentence, including adverbs like heute.

So this order is very natural:

• Mein Browser öffnet heute ...

You can also move heute for emphasis:

• Heute öffnet mein Browser zu viele Seiten gleichzeitig.

That is also correct, but now heute is emphasized more strongly.

Because German often places adverbs like this later in the sentence, especially when they describe how the action happens.

Here, gleichzeitig means simultaneously / at the same time. It tells you how the pages are being opened.

Putting it at the end sounds natural:

• Mein Browser öffnet heute zu viele Seiten gleichzeitig.

It clearly applies to the action of opening the pages.

Yes, absolutely.

That sentence is correct and means the same thing overall. The main difference is emphasis:

• Mein Browser öffnet heute ... = neutral, starting with the subject
• Heute öffnet mein Browser ... = emphasizes today

This is a very common feature of German word order: you can move one element to the first position for emphasis, but the finite verb still stays in second position.

Often yes, but gleichzeitig is the more compact and natural choice here.

Both can mean at the same time, but:

• gleichzeitig is a single adverb
• zur gleichen Zeit is a longer phrase

In this sentence, gleichzeitig sounds smoother and more idiomatic:

• Mein Browser öffnet heute zu viele Seiten gleichzeitig.

It expresses that the pages are being opened all at once, not one after another.