Jos biojäte ja sekajäte menevät samaan pussiin, ne täytyy lajitella uudestaan.

Because the subject is plural in meaning: biojäte ja sekajäte = biowaste and mixed waste.

In Finnish, when two nouns are joined with ja (and), the verb is normally plural:

• Biojäte menee pussiin. = The biowaste goes into the bag.
• Sekajäte menee pussiin. = The mixed waste goes into the bag.
• Biojäte ja sekajäte menevät pussiin. = The biowaste and mixed waste go into the bag.

So menevät is the 3rd person plural form of mennä.

Because the sentence expresses movement into the bag.

Finnish often shows location and movement with different case endings:

• pussissa = in the bag (location)
• pussiin = into the bag (movement)

Here the waste goes into one bag, so pussiin is needed.

The word samaan comes from sama (same) and matches the case of pussiin:

• sama pussi = the same bag
• samaan pussiin = into the same bag

So both words reflect the idea into the same bag.

It is the illative case, which often means into.

The basic form is pussi (bag), and the illative form is:

• pussi → pussiin

This case is commonly used with verbs of movement, such as:

• mennä pussiin = to go into the bag
• laittaa pussiin = to put into the bag

Because samassa means in the same place, while samaan means into the same place.

Compare:

• samassa pussissa = in the same bag
• samaan pussiin = into the same bag

Since the verb mennä here describes movement, Finnish uses the into form, not the in form.

Jos means if.

It introduces a condition:

• Jos biojäte ja sekajäte menevät samaan pussiin... = If biowaste and mixed waste go into the same bag...

This is a very common Finnish pattern:

• Jos sataa, jäämme kotiin. = If it rains, we stay home.
• Jos ehdit, soita. = If you have time, call.

In Finnish, the verb after jos is often in the present tense even when English might use future meaning.

Because the sentence has two clauses, and the first clause is a jos clause.

Finnish normally uses a comma before the main clause in sentences like this:

• Jos tulet ajoissa, ehdimme bussiin.
• Jos biojäte ja sekajäte menevät samaan pussiin, ne täytyy lajitella uudestaan.

So the comma is standard punctuation here.

Ne refers back to biojäte ja sekajäte.

So it means they:

• biojäte ja sekajäte = biowaste and mixed waste
• ne = they

In Finnish, ne can refer to things, not just people. That is completely normal.

So here:

• ne täytyy lajitella uudestaan = they have to be sorted again

Yes. Very commonly.

In standard Finnish:

• hän = he/she
• he = they (people)
• se = it
• ne = they (things, animals, or sometimes people in everyday speech)

So in this sentence, ne is exactly what you would expect, because the referents are types of waste, not people.

Because täytyy is part of an impersonal necessity structure.

In Finnish, täytyy often stays in the 3rd person singular, even if the thing that must be done is plural:

• Minun täytyy mennä. = I must go.
• Sinun täytyy odottaa. = You must wait.
• Ne täytyy lajitella. = They must be sorted.

This structure does not work like a normal agreeing verb in English. The important pattern is:

• [something/someone] täytyy + infinitive

So ne täytyy lajitella is a normal and idiomatic way to say they have to be sorted.

Because after täytyy, Finnish uses the basic infinitive of the main verb.

So the structure is:

• täytyy lajitella = must be sorted / must sort

Here lajitella means to sort.

Examples:

• Täytyy lähteä. = Must leave.
• Roskat täytyy viedä ulos. = The trash must be taken out.
• Ne täytyy lajitella uudestaan. = They must be sorted again.

So lajitella is in the dictionary/basic infinitive form because that is what täytyy requires.

Because Finnish often leaves this kind of thing more implicit than English does.

Literally, ne täytyy lajitella is something like they must sort / they must be sorted, but in context only be sorted makes sense. Waste does not sort anything itself, so the natural English translation is passive:

• They must be sorted again.

Finnish often uses this necessity structure without an explicit passive form when the meaning is clear from context.

Uudestaan means again or anew.

So:

• lajitella uudestaan = sort again / re-sort

It suggests that the sorting has to be done one more time because it was done wrongly or the items got mixed together.

A close synonym is uudelleen. In many situations, they are interchangeable:

• Ne täytyy lajitella uudestaan.
• Ne täytyy lajitella uudelleen.

Both mean about the same thing.

Because they are the subject of the first clause.

The clause is:

• biojäte ja sekajäte menevät samaan pussiin

Here biojäte and sekajäte are the things doing the action of going, so they appear in the nominative/basic form.

Also, these are compound nouns:

• bio + jäte = biowaste / organic waste
• seka + jäte = mixed waste / general waste

Not necessarily.

Finnish often does not use articles like a or the, because Finnish has no articles. So samaan pussiin can mean something like:

• into the same bag
• into one and the same bag

The exact English wording depends on context. Finnish does not force you to choose between a and the in the way English does.

Because Finnish has no articles.

So where English needs:

• the bag
• a bag
• the waste

Finnish simply uses the noun form and lets context do the work:

• pussi
• jäte

In this sentence, English has to choose something like the same bag, but Finnish expresses that meaning without articles.

Yes.

The sentence uses the present tense, but it expresses a general instruction or rule:

• Jos biojäte ja sekajäte menevät samaan pussiin, ne täytyy lajitella uudestaan.

This means something like:

• If biowaste and mixed waste end up in the same bag, they have to be sorted again.

Finnish often uses the present tense for general truths, rules, and instructions.

Yes, to some extent.

Finnish word order is fairly flexible, though the given version is natural and clear:

• Jos biojäte ja sekajäte menevät samaan pussiin, ne täytyy lajitella uudestaan.

You could also say:

• Ne täytyy lajitella uudestaan, jos biojäte ja sekajäte menevät samaan pussiin.

The meaning stays the same, but the focus changes slightly. The original version is a very normal way to present a condition first and then the consequence.

Here it is basically literal: to go into the same bag.

In context, it means the two types of waste are put together or end up together in one bag.

So it can describe either:

• someone putting them in the same bag, or
• them ending up in the same bag

Finnish often uses mennä in this practical, everyday way.

Yes, but the sentence you have is very natural as it is.

For example, Finnish can express obligation in other ways too, such as with pitää or on lajiteltava, but täytyy lajitella is a common and straightforward choice.

So for a learner, the key pattern to remember is:

• Jos ..., ne täytyy lajitella uudestaan.

It is idiomatic, normal Finnish.