Ne volim pretpostavljati što ljudi misle; radije ih izravno pitam.

This is about aspect, which is very important in Croatian.

• pretpostavljati = imperfective
• pretpostaviti = perfective

Here the speaker is talking about a general habit or attitude: they do not like assuming in general. For that, Croatian normally uses the imperfective verb, so pretpostavljati fits naturally.

If you used pretpostaviti, it would sound more like to make one completed assumption in a specific instance.

So:

• Ne volim pretpostavljati... = I don’t like assuming / making assumptions
• pretpostaviti would be less natural here for the general meaning

This is a very common Croatian pattern.

• volim = I like / I love
• ne volim = I don’t like
• pretpostavljati = to assume

So ne volim pretpostavljati literally means I don’t like to assume, and in natural English it often becomes I don’t like assuming.

Croatian often uses voljeti / ne voljeti + infinitive where English can use either:

• I like to read
• I like reading

Both ideas can be expressed with this structure in Croatian.

što ljudi misle literally means what people think.

Breakdown:

• što = what
• ljudi = people
• misle = think

Here što introduces an indirect question or embedded clause, just like English what in I wonder what people think or I don’t like assuming what people think.

So the structure is very close to English:

• pretpostavljati što ljudi misle
• to assume what people think

In everyday speech, you may also hear šta instead of što, especially in colloquial language.

Because ljudi is plural, the verb must also be plural.

• misliti = to think
• mislim = I think
• misliš = you think
• misli = he/she/it thinks
• mislimo = we think
• mislite = you all / formal you think
• misle = they think

So:

• ljudi misle = people think

Croatian uses a full clause here, not an infinitive, because after što you need a finite verb:

• što ljudi misle = what people think

Radije means rather, preferably, or instead.

In this sentence, it expresses preference between two actions:

• not assuming
• asking people directly

So radije ih izravno pitam means something like:

• I’d rather ask them directly
• instead, I ask them directly
• I prefer to ask them directly

A related form is radije bih, which is often used for a more explicit I would rather:

• Radije bih pitao/pitala. = I’d rather ask.

But in your sentence, radije pitam already sounds perfectly natural.

Ih means them.

It is the accusative plural clitic pronoun, used here as the direct object of pitam:

• pitati koga? = to ask whom?
• pitam ih = I ask them

In this sentence, ih refers back to ljudi = people.

So:

• ljudi = people
• ih = them

Croatian often avoids repeating the noun if it is already clear.

Because ih is a clitic, and Croatian clitics usually go in the second position of the clause.

In:

• radije ih izravno pitam

the first main element is radije, and the clitic ih comes right after it.

This is very normal Croatian word order.

A few useful comparisons:

• Radije ih izravno pitam. = very natural
• Pitam ih izravno. = also natural
• Njih pitam izravno. = possible, but now njih is stressed/emphatic: them I ask directly

So the placement of ih is not random; it follows a common clitic rule.

Izravno means directly.

So:

• izravno pitam = ask directly

Yes, direktno is also used and would be understood very easily:

• radije ih direktno pitam

Both are natural, but there is a slight difference in feel:

• izravno can sound a bit more standard or stylistically neutral
• direktno is also very common and everyday

So for a learner, both are useful, but izravno is an excellent standard choice.

Yes, Croatian word order is more flexible than English, but the different versions can sound more or less natural, or give different emphasis.

Your sentence:

• Radije ih izravno pitam.

Other possible versions:

• Ih radije izravno pitam. — less natural as a neutral sentence
• Pitam ih radije izravno. — possible, but less smooth in this context
• Radije pitam njih izravno. — puts stronger emphasis on them
• Izravno ih radije pitam. — possible, but marked

So the original order is good because it is both natural and balanced, and it respects normal clitic placement.

Yes, here ljudi means people.

This is the plural form related to čovjek:

• čovjek = person / man
• ljudi = people

So ljudi misle is simply people think.

The verb misle confirms that ljudi is plural, because misle means they think.

The semicolon separates two closely connected complete ideas:

• Ne volim pretpostavljati što ljudi misle
• radije ih izravno pitam

It shows a strong connection, but also a contrast:

• I don’t like doing X; instead, I prefer doing Y.

You could also see other punctuation or structures in real Croatian, for example:

• Ne volim pretpostavljati što ljudi misle, radije ih izravno pitam.
• Ne volim pretpostavljati što ljudi misle, nego ih radije izravno pitam.

But the semicolon is perfectly good and makes the contrast especially clear.