Op die website heb ik drie tabbladen open, maar ik weet niet meer in welk tabblad het formulier staat.

Because the sentence starts with Op die website, not with the subject ik.

In Dutch main clauses, the finite verb usually comes in second position. This is the V2 rule. So when you put another element first, the verb must come before the subject:

• Ik heb drie tabbladen open.
• Op die website heb ik drie tabbladen open.

So heb ik is not a special form here; it is just normal Dutch word order after something else has been placed first.

Dutch often uses op with websites, webpages, and online platforms:

• op een website
• op internet
• op Facebook

For an English speaker, this can feel a bit different, because English often uses on as well, but sometimes learners expect in because a website seems like a kind of space. In Dutch, op is the normal choice here.

So:

• op die website = on that website

Tabblad means tab, like a browser tab.

Its plural is tabbladen.

Examples:

• één tabblad = one tab
• drie tabbladen = three tabs

This is a common computer-related word in Dutch.

Both are possible, but they do not work in exactly the same way.

In this sentence, open is used like a result/state complement after hebben:

• Ik heb drie tabbladen open.
• literally: I have three tabs open.

This is very natural Dutch and matches English quite closely.

You can also say:

• Ik heb drie open tabbladen.

That means I have three open tabs. This is also correct, but it presents open more like a normal adjective directly describing the noun.

So:

• drie tabbladen open = I have three tabs open
• drie open tabbladen = I have three open tabs

The first version is especially common in everyday speech.

Niet meer means no longer, not anymore, or in this context often I don’t remember anymore.

So:

• ik weet niet meer = I no longer know / I don’t know anymore

It often suggests that the speaker used to know, but has forgotten.

Compare:

• Ik weet het niet. = I don’t know.
• Ik weet het niet meer. = I don’t know anymore / I no longer know.

That little meer adds the idea of a change over time.

Because tabblad is a het-word:

• het tabblad

For singular het-words, the interrogative determiner is welk:

• welk boek
• welk tabblad

For singular de-words, you use welke:

• welke tafel
• welke website

So:

• in welk tabblad = in which tab

Because the speaker is asking about one specific tab out of those three.

They have three tabs open, but the form is in only one of them. So Dutch uses the singular:

• drie tabbladen = three tabs
• in welk tabblad = in which tab

English does exactly the same:

• I have three tabs open, but I don’t know in which tab the form is.

Because in welk tabblad het formulier staat is an indirect question (embedded question), and in Dutch subordinate clauses the finite verb usually goes to the end.

Compare:

Direct question:

• In welk tabblad staat het formulier?

Indirect question:

• Ik weet niet meer in welk tabblad het formulier staat.

So the final position of staat is standard subordinate-clause word order.

Dutch often uses posture/location verbs such as staan, liggen, and zitten where English often simply uses be.

Here staat is very natural for something that appears somewhere on a page or screen:

• Het formulier staat in dat tabblad.

It suggests the form is there / appears there / is located there.

Using is is sometimes possible in other contexts, but staat is the idiomatic choice here.

English usually says:

• the form is in that tab

Dutch often prefers:

• het formulier staat in dat tabblad

Sometimes in informal Dutch, yes, but staat is better here.

• staan is common for text, information, buttons, forms, and things shown on a page or screen
• zitten can be used colloquially for where something is located, especially when searching for it, but it is less neutral here

So:

• Waar staat het formulier? = standard/natural
• Waar zit het formulier? = possible in casual speech, but less ideal in this sentence

For a learner, staat is the safest choice.

Because formulier is a het-word:

• het formulier

That matters for things like:

• the article: het
• the pronoun: het
• related question words in some structures

For example:

• Het formulier staat daar.
• Waar staat het formulier?

Unfortunately, whether a noun is a de-word or het-word often just has to be learned.

It is understandable, but it is less natural than staat.

Dutch often prefers staan for things shown on a screen, page, or document. So native speakers would usually say:

• Ik weet niet meer in welk tabblad het formulier staat.

If you say is, people will probably understand you, but it sounds less idiomatic.

It has two main parts joined by maar:

1. Op die website heb ik drie tabbladen open
2. maar ik weet niet meer in welk tabblad het formulier staat

A rough structure is:

• Op die website = fronted adverbial
• heb ik = verb + subject because of V2 word order
• drie tabbladen open = object + complement
• maar = but
• ik weet niet meer = main clause
• in welk tabblad het formulier staat = indirect question / subordinate clause

So the sentence combines:

• normal main clause word order
• inversion after a fronted phrase
• a subordinate clause with the verb at the end

That makes it a very useful example of everyday Dutch grammar.