Meddelandet säger att paketet kan innehålla mat, så vi måste vara försiktiga.

Because they are in the definite singular form: the message and the package.

In Swedish, the definite article is usually attached to the end of the noun instead of being a separate word like English the.

• ett meddelande = a message

• meddelandet = the message

• ett paket = a package
• paketet = the package

Both meddelande and paket are ett-words (neuter nouns), so their definite singular ending is -et.

Here att means that.

So:

• Meddelandet säger att ... = The message says that ...

It introduces a subordinate clause, just like English that does.

In learner-friendly terms:

• main clause: Meddelandet säger
• subordinate clause: att paketet kan innehålla mat

Because after a modal verb like kan (can), Swedish uses the bare infinitive without att.

So:

• kan innehålla = can contain
• not kan att innehålla

This is similar to English:

• can contain
• not can to contain

Other common Swedish modal verbs work the same way:

• måste vara = must be
• vill gå = want to go
• ska komma = will/is going to come

Because mat here is being used as an uncountable/mass noun, like food in English.

So Swedish says:

• innehålla mat = contain food

not usually:

• innehålla en mat
• innehålla ett mat

That would sound wrong, just like contain a food would usually sound wrong in English unless you meant a specific type or item.

Because the adjective agrees with the subject vi (we), which is plural.

After vara (to be), Swedish adjectives often change form depending on number and sometimes gender.

Compare:

• Jag är försiktig. = I am careful.
• Det är försiktigt. = It is careful / cautious.
• Vi är försiktiga. = We are careful.

Since vi is plural, the adjective takes the plural form försiktiga.

Because försiktig/försiktiga is an adjective, so Swedish needs the verb vara (to be) with it.

• vara försiktig/försiktiga = to be careful

This works much like English:

• We must be careful.

You cannot normally just say vi måste försiktiga, because the adjective needs a verb.

Because så here works like a coordinating conjunction meaning so, and after it Swedish can continue with normal main-clause word order:

• så vi måste vara försiktiga = so we must be careful

That means:

• vi = subject
• måste = verb

This is different from some other sentence connectors that trigger inversion. For example:

• Därför måste vi vara försiktiga. = Therefore we must be careful.

After därför, the verb comes before the subject in normal main-clause Swedish word order. But with så meaning so, the non-inverted order så vi måste ... is very natural.

Sometimes in spoken or informal Swedish, att can be omitted after verbs like säga. So you may hear:

• Meddelandet säger paketet kan innehålla mat.

But for learners, and in standard written Swedish, it is better to keep att:

• Meddelandet säger att paketet kan innehålla mat.

So the safest rule is: use att after säga when introducing a clause like this.

Here så means so, in the sense of therefore / as a result.

So the logic is:

• The message says that the package may contain food
• so / therefore
• we must be careful

Be aware that så has many other meanings in Swedish, depending on context, such as:

• so
• such
• then
• like that

But in this sentence, it is clearly the result meaning: so.

Because måste is the direct and natural translation of must.

• vi måste vara försiktiga = we must be careful

It expresses necessity or strong obligation.

Swedish also has softer ways to express similar ideas, for example:

• vi bör vara försiktiga = we should be careful
• vi behöver vara försiktiga = we need to be careful

But måste is the strongest and most direct match for English must.

Here are the main dictionary forms:

• meddelandet → meddelande = message
• säger → säga = to say
• att = that
• paketet → paket = package
• kan = modal verb, dictionary form often listed as kunna = can / to be able to
• innehålla = to contain
• mat = food
• så = so
• vi = we
• måste = modal verb, dictionary form måste or sometimes connected with måsta in grammatical discussions, but learners usually just memorize måste
• vara = to be
• försiktiga → försiktig = careful

This is useful because Swedish words often appear in inflected forms in real sentences.

In this sentence, kan is often best understood as may in English:

• paketet kan innehålla mat = the package may contain food

That is because it expresses possibility, not ability.

So although kan often translates as can, the actual meaning depends on context:

• Jag kan simma. = I can swim. → ability
• Det kan regna i morgon. = It may rain tomorrow. → possibility

Here it is clearly the possibility meaning.