Giften er i flasken.

In Norwegian, the definite article is usually attached to the end of the noun instead of being a separate word in front.

• gift = poison
• giften = the poison

This is very common in Norwegian:

• bil = car → bilen = the car
• bok = book → boka/boken = the book
• flaske = bottle → flasken = the bottle

So giften simply means the poison.

For the same reason: Norwegian usually marks definiteness by adding an ending to the noun.

• en flaske = a bottle
• flasken = the bottle

English uses a separate word (the), but Norwegian often uses a suffix instead.

So:

• i en flaske = in a bottle
• i flasken = in the bottle

Er is the present tense of å være, which means to be.

So:

• er = is / are / am, depending on context

In this sentence:

• Giften er i flasken = The poison is in the bottle

Norwegian does not change the verb for different persons in the present tense:

• jeg er = I am
• du er = you are
• han er = he is
• de er = they are

This is the normal word order for a simple statement in Norwegian:

subject + verb + other information

Here:

• Giften = subject
• er = verb
• i flasken = prepositional phrase

So the structure is:

The poison + is + in the bottle

This is very similar to English word order.

I means in.

So:

• i flasken = in the bottle

It is a very common preposition in Norwegian. For example:

• i huset = in the house
• i bilen = in the car
• i byen = in the city

Norwegian uses different prepositions depending on the relationship between things.

Here, i is used because the poison is inside the bottle.

• i = in / inside
• på = on / on top of / in some expressions

So:

• Vannet er i glasset = The water is in the glass
• Boken er på bordet = The book is on the table

Because poison is contained inside a bottle, i is the natural choice.

Yes, gender matters in Norwegian because it affects articles and some endings.

• gift is a neuter noun: et giftstoff is different vocabulary, but gift as poison is commonly treated as masculine in the phrase giften in standard usage here.
• flaske is a feminine noun, but in Bokmål many feminine nouns can also take masculine forms.

That is why you often see:

• en flaske = a bottle
• flasken = the bottle

Even though flaske is traditionally feminine, Bokmål often allows the common-gender-style ending -en instead of feminine -a.

So flaska may also exist in some styles, but flasken is very common and completely correct in Bokmål.

Yes, in many varieties of Bokmål, flaska is also possible.

So you may see:

• Giften er i flasken
• Giften er i flaska

Both mean the same thing: The poison is in the bottle.

The difference is mainly style and noun gender preference:

• flasken = common in more conservative or gender-neutral Bokmål
• flaska = feminine form, also very common

That would be:

Giften er i en flaske.

Compare:

• i en flaske = in a bottle
• i flasken = in the bottle

So the difference is:

• en flaske = indefinite
• flasken = definite

A rough English-friendly guide is:

GIF-ten er i FLASK-en

A few helpful notes:

• g in giften is a hard g, like in go
• i is pronounced like ee
• er is often said quickly, almost like a short ær or e(r) depending on accent
• flasken has two syllables after flask-: FLASK-en

Norwegian pronunciation varies by dialect, so the exact sound can differ, but that rough pattern will help.

Yes, it is a perfectly natural and correct Norwegian sentence.

It is a simple, standard statement with very normal grammar:

• definite noun as subject: giften
• present tense verb: er
• prepositional phrase: i flasken

It sounds like a straightforward factual statement: The poison is in the bottle.

Yes, and this is an important vocabulary point.

Gift can mean:

• poison
• married in the adjective form gift

For example:

• Han er gift. = He is married.
• Giften er i flasken. = The poison is in the bottle.

In your sentence, giften clearly means the poison, because it is a noun with the definite ending -en and it fits the context of being in a bottle.