Ubuduće ću se truditi da ne nosim svakakve sitnice u torbi, jer mi onda ni običan ključ ne stane u džep.

Ubuduće means from now on, from this point forward, or sometimes in the future depending on context. Here it clearly means from now on: the speaker is making a resolution about future behavior.

In standard Croatian, it is normally written as one word: ubuduće.

Croatian often leaves out subject pronouns when they are already clear from the verb form.

Here, ću already tells you the subject is I:

• ću = I will

So Ja ću se truditi... is possible, but ja is unnecessary unless you want extra emphasis.

Ću is an unstressed word, a clitic, and clitics in Croatian usually go in the second position in the clause.

So:

• Ubuduće = first stressed element
• ću se = clitic cluster placed right after it

That is why you get:

• Ubuduće ću se truditi...

A more expanded version could be:

• Ja ću se ubuduće truditi...

Both are fine, but the original word order is very natural.

Truditi se means to try, to make an effort, or to do one’s best.

Yes, se is part of the verb here. It is not optional. In this expression, you learn the verb as:

• truditi se = to make an effort / to try

So:

• truditi se = correct
• truditi by itself would not mean the same thing in normal usage here

Croatian very often uses da + present tense after verbs expressing effort, intention, desire, and similar ideas.

So:

• truditi se da ne nosim = to try not to carry

For an English speaker, this may feel unusual because English prefers to try not to carry with an infinitive. Croatian often prefers a da-clause in this kind of sentence.

An infinitive can sometimes be possible in related structures, but da ne nosim is completely natural and common.

After da, Croatian normally uses a present-tense form, even when the meaning is future or intended.

So:

• ću se truditi da ne nosim...

literally looks like:

• I will try that I do not carry...

but the real meaning is:

• I will try not to carry...

This is a normal Croatian pattern. The present form nosim does not mean present time here; it is part of the da + present construction.

Sitnice means small things, little items, bits and pieces, or odds and ends.

Svakakve adds the idea of:

• all kinds of
• various
• random
• sometimes even unnecessary miscellaneous

So svakakve sitnice means something like:

• all sorts of little things
• random small items
• all kinds of odds and ends

There is a slightly dismissive tone: the speaker is talking about clutter, not important objects.

Because it is the direct object of nosim.

The verb nositi takes an object in the accusative case, and here the object is plural feminine:

• sitnice = accusative plural
• svakakve matches it in case, number, and gender

So the whole phrase agrees as:

• svakakve sitnice = all sorts of little things

This is a very useful case difference.

• u torbi uses the locative because it means in the bag as a location
• u džep uses the accusative because with stati the meaning is fit into the pocket

So:

• nosim ... u torbi = I carry them in the bag
• ključ ne stane u džep = the key does not fit into the pocket

If you said u džepu, that would describe something already being in the pocket, not the idea of fitting into it.

Mi is the dative form meaning to me or for me.

In this sentence, English would usually express the same idea with my:

• ne stane mi u džep = it doesn’t fit in my pocket

So mi marks the person affected, and English often translates that with a possessive:

• mi here ≈ my, in natural English

This is a very common Croatian pattern.

The present tense ne stane is used here to describe a typical result or general consequence:

• if I carry all those little things, then even an ordinary key doesn’t fit in my pocket

So it has a kind of whenever that happens meaning.

If you said neće stati, that would sound more like a specific future event:

• then it won’t fit

The original sentence uses the present because it sounds more like a habitual or predictable outcome.

With a negative verb, ni means not even.

So:

• ni običan ključ ne stane = not even an ordinary key fits

This is a common Croatian pattern:

• ni
  • negative verb

You will often see:

• ni ... ne ...

It emphasizes that even something very basic or small fails to fit.

Yes. Običan ključ literally means ordinary key or normal key.

The point is emphatic: the bag or pocket situation is so bad that even a simple, everyday key does not fit.

Grammatically:

• običan = adjective, masculine singular
• ključ = noun, masculine singular
• together they are the subject of ne stane

Jer means because.

It introduces the reason:

• Ubuduće ću se truditi... = what the speaker intends to do
• jer... = why

The comma before jer is standard because it introduces a subordinate clause:

• ..., jer mi onda ni običan ključ ne stane u džep.

So the second part explains the motivation for the first part.