Om jag tappar bort mitt bokmärke igen, måste jag bläddra från början för att hitta rätt sida.

Om means if and introduces a condition.

Here, the speaker means something like: this might happen, and if it does, then I have to do something. That is why om is the natural choice.

När means when and usually sounds more definite, as if the speaker expects it to happen rather than just presenting it as a possibility.

So:

• Om jag tappar bort mitt bokmärke igen ... = If I lose my bookmark again ...
• När jag tappar bort mitt bokmärke igen ... would sound more like When I lose my bookmark again ..., which suggests the speaker thinks it is likely or inevitable.

Yes. Tappa bort is a very common Swedish particle verb.

The basic verb tappa can mean drop or sometimes lose, but tappa bort specifically means misplace or lose track of something.

So:

• tappa = drop, lose
• tappa bort = lose/misplace something so you cannot find it

Examples:

• Jag tappade glaset. = I dropped the glass.
• Jag tappade bort nycklarna. = I lost/misplaced the keys.

A native English speaker may expect one single verb for lose, but Swedish often uses a verb + particle combination here.

Because bokmärke is an ett-word.

In Swedish, possessives change depending on the gender and number of the noun:

• min for en-words
• mitt for ett-words
• mina for plural nouns

Since it is:

• ett bokmärke = a bookmark

the possessive form is:

• mitt bokmärke = my bookmark

Compare:

• en bok → min bok
• ett bokmärke → mitt bokmärke

That is the most natural word order in this sentence.

Igen means again, and here it applies to the whole action of losing the bookmark again. In normal Swedish word order, adverbs like igen often come after the object in this kind of clause:

• Om jag tappar bort mitt bokmärke igen ...

This sounds natural and neutral.

Other placements are sometimes possible, but they usually sound more marked, formal, or less idiomatic. So for everyday Swedish, this is a good pattern to copy.

This is because Swedish follows the V2 rule in main clauses: the finite verb normally comes in the second position.

The first part, Om jag tappar bort mitt bokmärke igen, is a subordinate clause placed at the front. Once that whole clause takes the first position, the main clause must begin with the finite verb:

• ... måste jag bläddra ...

So the structure is:

1. fronted clause
2. finite verb
3. subject

Compare:

• Jag måste bläddra från början.
• Om jag tappar bort mitt bokmärke igen, måste jag bläddra från början.

This inversion is very common in Swedish.

Because måste is a modal verb, and modal verbs are followed by the bare infinitive in Swedish.

So you say:

• måste bläddra
• kan komma
• vill läsa
• ska gå

not:

• måste att bläddra
• kan att komma

This is similar to English, where you say must read, not must to read.

Bläddra means something like flip through, leaf through, or turn pages.

Very often, when you mention what you are flipping through, Swedish uses i:

• bläddra i en bok = flip through a book
• bläddra i en tidning = leaf through a magazine

In this sentence, the object is left out because it is already understood from the context. The speaker has lost a bookmark, so it is obvious that they have to flip through the book or pages.

So the sentence does not need i boken, but you could say it if you wanted to make it explicit:

• ... måste jag bläddra i boken från början ...

Because från början means from the beginning / from the start, which fits this situation.

The speaker means they have to start turning pages from page 1 or from the start of the book.

• från början = from the beginning
• i början = in the beginning / at first

So:

• Jag måste bläddra från början. = I have to flip through from the beginning.
• I början var boken långsam. = At first, the book was slow.

This is an important distinction in Swedish.

Here för att means in order to and introduces a purpose.

So:

• måste jag bläddra från början för att hitta rätt sida means
• I have to flip through from the beginning in order to find the right page

This is a very common structure:

• Jag gick hem för att vila. = I went home to rest.
• Hon öppnade boken för att läsa. = She opened the book to read.

Since the subject is the same in both parts, Swedish uses för att + infinitive very naturally here.

Here it means the correct page.

The word sida can mean both side and page, so context matters. Because the sentence is about a bookmark, the meaning is clearly page.

Also, rätt here means correct, not the opposite of left.

So:

• rätt sida = the correct page
• högra sidan = the right-hand side/page

That is a very common point of confusion for English speakers.

This is an idiomatic Swedish pattern. With words like rätt and fel, Swedish often leaves out the article when talking about finding, choosing, or going to the correct thing.

So these are very natural:

• hitta rätt sida = find the right page
• ta rätt buss = take the right bus
• komma till rätt plats = get to the right place
• välja rätt bok = choose the right book

English often uses the here, but Swedish does not always need an article in these expressions.

If you said den rätta sidan, that would sound more specific, more formal, or stylistically different. In this sentence, rätt sida is the normal everyday choice.