Learn Hardcore Croatian: Ponekad sumnjam jesam li dovoljno iskrena kad govorim hrvatski. - Sometimes I doubt whether I am sincere enough when I speak Croatian.

Both are grammatically correct.
Putting ponekad at the beginning is more natural and puts more emphasis on how often this happens, while putting it after sumnjam slightly emphasizes the doubting itself.

You may also see near‑synonyms:

nekad(a) – sometimes, once (can also mean once, in the past depending on context)
katkad – sometimes (a bit more formal/literary)

In this context, ponekad is the safest and most neutral choice.

Why is sumnjam followed directly by a clause (jesam li…) and not by u + something?

The verb sumnjati (to doubt) can be used in a few ways:

sumnjati u + accusative – to doubt someone/something
- Sumnjam u sebe. – I doubt myself.
- Sumnjam u to. – I doubt that.
sumnjati da + clause – to doubt that…
- Sumnjam da je iskrena. – I doubt that she is sincere.
sumnjati + indirect question – to doubt whether/if…
- Sumnjam jesam li dovoljno iskrena. – I doubt whether I am sincere enough.

In your sentence, sumnjam is followed by an indirect yes/no question (jesam li dovoljno iskrena), which in English we translate with whether or if. This is very natural in Croatian.

You could also lengthen it to:

Ponekad sumnjam u to jesam li dovoljno iskrena…

This is a bit heavier; your original sentence is smoother and more idiomatic.

What is jesam li doing here? Why not just da sam dovoljno iskrena?

Jesam li introduces an indirect yes/no question and corresponds to English whether I am (or if I am):

jesam li dovoljno iskrena → whether I am sincere enough

So the structure is:

Ponekad sumnjam (I sometimes doubt)
jesam li dovoljno iskrena (whether I am sincere enough)
kad govorim hrvatski (when I speak Croatian).

If you used da, you’d get a different structure:

Sumnjam da sam dovoljno iskrena. – I doubt that I am sincere enough.

Both are possible, but:

…sumnjam da sam dovoljno iskrena = a simple content clause (that).
…sumnjam jesam li dovoljno iskrena = an indirect question (whether/if).

In your exact sentence, jesam li is more precise because the doubt is about whether you reach some standard, not just a flat statement you contest.

Why is it jesam li, not sam li or da li sam?

The verb biti (to be) has full forms and clitic (short) forms:

1st person singular: jesam (full), sam (clitic)
2nd singular: jesi, si
3rd singular: jest / je

In yes/no questions with inversion and li, standard Croatian uses the full forms for 1st person singular:

Jesam li dovoljno iskrena? – Am I sincere enough?
Ne znam jesam li dovoljno iskrena. – I don’t know whether I am sincere enough.

Using sam li is non‑standard or dialectal in this structure.

About da li:

In indirect questions, jesam li is more standard and neutral:
- Ne znam jesam li dovoljno iskrena.
Da li sam dovoljno iskrena? is very common in speech as a direct yes/no question, but in a sentence like yours, most teachers and grammar books would recommend jesam li, not da li sam, in the subordinate clause:
- Preferred: …sumnjam jesam li dovoljno iskrena…
- Less standard: …sumnjam da li sam dovoljno iskrena…

What exactly is li, and where must it go in the sentence?

Li is a question particle used to form yes/no questions, both direct and indirect.

Key rules:

Li immediately follows the first stressed word in its clause.
In your clause, the first stressed word is jesam, so we get:
- jesam li dovoljno iskrena
You cannot say:
- li jesam dovoljno iskrena ✗
- jesam dovoljno li iskrena ✗
It stays inside its own clause; it doesn’t move to the start of the whole sentence.

So in your full sentence:

Main clause: Ponekad sumnjam
Subordinate clause: jesam li dovoljno iskrena kad govorim hrvatski

The li belongs to jesam li…, not to Ponekad sumnjam.

Why is it jesam li dovoljno iskrena, and not jesam li iskrena dovoljno?

In Croatian, degree adverbs like dovoljno (enough), jako (very), prilično (quite) normally stand before the adjective they modify:

dovoljno iskrena – sincere enough
jako iskrena – very sincere
prilično iskrena – quite sincere

Putting the adverb after the adjective (iskrena dovoljno) is possible only in very marked, poetic, or special stylistic contexts. In everyday speech, dovoljno iskrena is the natural order.

So:

jesam li dovoljno iskrena ✓ (normal)
jesam li iskrena dovoljno ✗ (sounds odd/wrong in normal speech)

Why is the adjective iskrena and not iskren?

Adjectives in Croatian agree in gender, number, and case with the noun (or implied noun) they describe.

Here, iskrena describes the speaker (I). The speaker is assumed to be female, so:

masculine: iskren
feminine: iskrena
neuter: iskreno

Thus:

A man would say:
Ponekad sumnjam jesam li dovoljno iskren kad govorim hrvatski.
A woman says:
Ponekad sumnjam jesam li dovoljno iskrena kad govorim hrvatski.

If more people are included:

masculine (mixed or all male) plural: iskreni
feminine plural: iskrene
- Ponekad sumnjamo jesmo li dovoljno iskreni/iskrene…

What does dovoljno mean here, and what kind of word is it?

Dovoljno here means enough / sufficiently and functions as a degree adverb modifying the adjective iskrena:

dovoljno iskrena – sincere enough

As an adverb, it is invariable (it doesn’t change for gender/number).

Note: dovoljno can also be an adjective or pronoun in other contexts (sufficient, enough), where it does change form, e.g.:

dovoljna količina – a sufficient quantity (feminine)
dovoljni razlozi – sufficient reasons (masc. plural)

But in dovoljno iskrena it’s an adverb, and it stays dovoljno regardless of who is speaking.

What is the difference between kad and kada in kad govorim hrvatski?

Kad and kada both mean when in this temporal sense.

kad govorim hrvatski
kada govorim hrvatski

They are interchangeable here.
Differences:

kad is shorter and very common in everyday speech and writing.
kada is a bit more formal or emphatic, and often used in careful writing or to avoid ambiguity.

There is no change in meaning in your sentence if you use kada instead of kad:

Ponekad sumnjam jesam li dovoljno iskrena kada govorim hrvatski.

Could I use ako instead of kad (ako govorim hrvatski)?

Generally no – it would change the meaning.

kad govorim hrvatski – when I speak Croatian (whenever I happen to be speaking it)
ako govorim hrvatski – if I speak Croatian (on the condition that I speak it / in case I speak it)

Your sentence describes something that happens whenever you speak Croatian (habitual situation), so kad is correct:

Ponekad sumnjam jesam li dovoljno iskrena kad govorim hrvatski.

Ako would sound like you only maybe speak Croatian, and if that happens, then you doubt yourself – a different nuance.

What tense/aspect is govorim, and why is it used here?

Govorim is:

present tense
of the imperfective verb govoriti (to speak).

In Croatian, the present tense of an imperfective verb is used for:

current actions: Sad govorim. – I’m speaking now.
habitual/repeated actions: Kad govorim hrvatski, sumnjam… – When I (habitually) speak Croatian, I doubt…

In your sentence, kad govorim hrvatski refers to situations whenever you speak Croatian, so the present tense, imperfective is exactly what you want.

Why is it kad govorim hrvatski and not kad ja govorim hrvatski?

Croatian is a pro‑drop language: subject pronouns (ja, ti, on, ona…) are usually omitted because the verb ending already shows the person:

govorim clearly indicates I speak.

So:

Kad govorim hrvatski… – When I speak Croatian…
Kad ja govorim hrvatski… – When I speak Croatian…

You add ja mainly for emphasis or contrast, for example:

Kad ja govorim hrvatski, svi me ispravljaju.
(When I speak Croatian, everyone corrects me – implying maybe others don’t get corrected.)

What exactly is hrvatski here – an adjective or a noun, and why is there no word for language?

Hrvatski is originally an adjective meaning Croatian (related to Croatia).

However, with verbs like govoriti (to speak), učiti (to learn), znati (to know), Croatian typically uses just the adjective to mean the language:

Govoriš li hrvatski? – Do you speak Croatian?
Učim hrvatski. – I’m learning Croatian.
Znam hrvatski. – I know Croatian.

You can also say hrvatski jezik (Croatian language), but that is more formal or explicit. In everyday speech, just hrvatski is normal.

Grammatically, in govorim hrvatski:

hrvatski functions like an object (historically an accusative of the adjective used as a noun), essentially meaning Croatian (language).
This same pattern works for other languages: engleski, španjolski, njemački, etc.

So kad govorim hrvatski literally means when I speak Croatian (language), even though jezik is not overtly said.