Learn Hardcore Croatian: Kad sam motiviran, lakše rješavam težak zadatak i ne vidim mane tog posla. - When I am motivated, I solve a difficult task more easily and I do not see the disadvantages of that job.

sam is the 1st person singular of biti (“to be”), so sam already implies ja.
Saying ja is only needed for emphasis (e.g. contrast: Ja sam motiviran, ali ti nisi. – I am motivated, but you are not).

Kad ja sam motiviran is ungrammatical in Croatian because the normal order is ja sam, not ja sam split up. You could say:

Kad sam ja motiviran… – When I am motivated… (emphasizing “I”)
Kad sam motiviran… – neutral, no special emphasis

What exactly is motiviran? Is it an adjective or some kind of participle?

Motiviran is originally a past passive participle (from motivirati – “to motivate”), but in practice it behaves like an adjective, meaning “motivated.”

It agrees in gender and number with the subject:

(ja) sam motiviran – I am motivated (male speaker)
(ja) sam motivirana – I am motivated (female speaker)
(mi) smo motivirani – We are motivated (mixed group or all male)
(mi) smo motivirane – We are motivated (all female group)

So in this sentence, the speaker is grammatically male (or using the generic masculine form).

Why is the verb sam before motiviran? Could it be Kad motiviran sam?

The usual, neutral order is auxiliary + complement, so:

Kad sam motiviran is the normal word order.

You can move things around for emphasis, but not every reordering is natural. In real language, you might hear:

Kad sam motiviran… – neutral
Kad sam stvarno motiviran… – When I am really motivated…
Motiviran sam kad… – I’m motivated when…

But Kad motiviran sam sounds unnatural. The auxiliary sam almost always directly follows the subject (explicit or implied) in such simple sentences.

Why is it rješavam, not riješim? Both mean “solve,” right?

Yes, but they differ in aspect, which is very important in Croatian.

rješavam – imperfective, ongoing / repeated / habitual action
riješim – perfective, a single, completed action

In this sentence, the speaker is talking about what generally happens when they’re motivated (a habit), so the imperfective rješavam is correct:

Kad sam motiviran, lakše rješavam težak zadatak.
= When I’m motivated, I tend to solve a difficult task more easily.

If you said:

Kad sam motiviran, lakše riješim težak zadatak.

it would focus more on a single act of solving: “When I’m motivated, I (successfully) solve a difficult task more easily (in that situation).”
Both can be acceptable depending on nuance, but for a general statement of habit, rješavam is more natural.

Why is težak zadatak singular in Croatian when English often says “difficult tasks” (plural)?

Croatian often uses a generic singular where English prefers a plural.

težak zadatak literally: “a difficult task”
But it can mean “a difficult task (in general)” or “difficult tasks in general” depending on context.

You could also say:

teške zadatke (plural accusative) – “difficult tasks”

So both are possible:

…lakše rješavam težak zadatak… – I (in general) solve a difficult task more easily.
…lakše rješavam teške zadatke… – I (in general) solve difficult tasks more easily.

The singular here doesn’t restrict it to one specific task; it’s a common Croatian way to talk about something in general.

What case is težak zadatak, and why does it look like the nominative?

Težak zadatak is in the accusative singular, because it’s the direct object of rješavam.

For masculine inanimate nouns in the singular, the nominative and accusative forms are identical:

Nominative: težak zadatak (subject)
Accusative: težak zadatak (object)

Example:

Ovaj težak zadatak je problem. – This difficult task is a problem. (nominative)
Rješavam ovaj težak zadatak. – I’m solving this difficult task. (accusative)

So the form doesn’t change, but the function in the sentence does.

How is lakše formed? Is it related to lako or lak?

Lakše is the comparative form of the adverb lako (“easily”).

Positive adverb: lako – easily
Comparative adverb: lakše – more easily

There’s also an adjective:

Positive adjective: lak – light / easy
Comparative adjective: lakši – lighter / easier

In your sentence, you need an adverb modifying the verb rješavam, so lakše is correct:

lakše rješavam – I solve (it) more easily.

If you were describing a noun, you’d use the adjective:

lak zadatak – an easy task
lakši zadatak – an easier task

What exactly does mane mean, and why is it plural?

Mana means “flaw, drawback, shortcoming, con, minus.”
Mane is its nominative or accusative plural form, but here it’s part of a genitive phrase (see below).

In this sentence:

mane = “drawbacks / flaws / cons” (plural)

So:

ne vidim mane ≈ “I don’t see (any) drawbacks / flaws.”

The plural just reflects that the speaker is thinking of multiple possible drawbacks to the job.

Why is it mane tog posla and not mane taj posao?

Because mana usually takes a genitive to specify “whose flaws / of what.”

mana (čega?) – a flaw of what?

So you get:

mane posla – flaws OF the job
mane tog posla – flaws OF that job

Here:

tog is the genitive singular of taj (“that”)
posla is the genitive singular of posao (“job”)

So mane tog posla literally means “the flaws of that job” or “that job’s drawbacks.”

Mane taj posao is incorrect, because taj posao is in the nominative or accusative, not in the needed genitive.

Could I say mane u tom poslu instead of mane tog posla? Does it change the meaning?

You can say mane u tom poslu, but it feels a bit different.

mane tog posla – the flaws / drawbacks of that job itself (its nature, conditions, pay, etc.)
mane u tom poslu – flaws in that job; more like things that exist within the context of that job (maybe problems that appear within it).

In many contexts they will overlap and both be understandable, but mane tog posla is the more straightforward and typical way to say “the drawbacks of that job.”

How does the negative ne vidim work? Does Croatian use double negatives like nijednu manu?

Ne before the verb makes it negative:

vidim – I see
ne vidim – I don’t see

In your sentence:

ne vidim mane tog posla – I don’t see the drawbacks of that job / I see no drawbacks of that job.

Croatian does allow (and often prefers) double negation with words like nijedan / nijedna / nijedno (“no / not a single”):

Ne vidim nijednu manu tog posla.
Literally: I don’t see not-a-single flaw of that job.
Natural English: I don’t see any flaw in that job / I see absolutely no flaw in that job.

So:

ne vidim mane tog posla – I don’t see (the) flaws of that job.
ne vidim nijednu manu tog posla – I don’t see a single flaw in that job (stronger emphasis).

Both are correct; the second is more emphatic.