1a and 1b are correct. Althought, Croatian grammar doesn't know formulation with "da li" (it should be jesam/jesi/je/jesmo/jeste/jesu li) but it's totally OK with "da li". With "je li" 1a should be: "Ne znam je li dao šešir.". 2a. Ne znam jesi li joj dao šešir (with "da li": Ne znam da li si joj dao šešir) 2b. Ne znam jesi li dao šešir (Ne znam da li si dao šešir) 3a. Ne znam kada si joj dao šešir. 3b. Ne znam kada si dao šešir. So, let's say that you'll use form with "da li" (since you've learned this way): - you ask should be "je" before "dao". It should, but it's not always word "je". In your examples (1a and 1b) "je" is present form for 3. person singular of verb "biti" (to be). Look how this "je" is changing with other persons: I don't know whether I gave (her) the hat Ne znam da li SAM (joj) dao šešir I don't know whether YOU (singular) gave (her) the hat Ne znam da li SI (joj) dao šešir I don't know whether HE gave (her) the hat / I don't know whether SHE gave (her) the hat Ne znam da li (joj) JE dao šešir / Ne znam da li (joj) JE dalA šešir I don't know whether WE gave (her) the hat Ne znam da li SMO (joj) dali šešir I don't know whether YOU (plural) gave (her) the hat Ne znam da li STE (joj) dali šešir I don't know whether THEY gave (her) the hat Ne znam da li SU (joj) dali šešir You may notice that word „joj“ is in all sentences (except in sentences with HE and SHE) set between present forms of verb „biti“ (sam, si, smo, ste, su) and main verb „dati“. In grammar is written that enclitic „je“ (which is form for present, 3. person of singular, verb „to be“) should be last of all enclitics in sentences; why – I don't know. This answer might be confusing, it wasn't my intention :)

About your questions: 1. Form "je li", "jesi li", and so on: With this form you must use longer version of present of the verb "to be" (biti). Longer version: JESAM (for I) JESI (you, singular) JE (for he/she/it) - this is the same like in short version JESMO (for we) JESTE (for you, plural) JESU (for they) Shorter version is: SAM (for I) SI (you, singular) JE (for he/she/it) SMO (for we) STE (for you, plural) SU (for they) I don't know whether I gave (her) the hat Ne znam JESAM LI (joj) dao šešir I don't know whether YOU (singular) gave (her) the hat Ne znam JESI LI (joj) dao šešir I don't know whether HE gave (her) the hat / I don't know whether SHE gave (her) the hat Ne znam JE LI (joj) dao šešir / Ne znam JE LI (joj) dalA šešir I don't know whether WE gave (her) the hat Ne znam JESMO LI (joj) dali šešir I don't know whether YOU (plural) gave (her) the hat Ne znam JESTE LI (joj) dali šešir I don't know whether THEY gave (her) the hat Ne znam JESU LI (joj) dali šešir 2. About "joj" and "je li" as you can see "joj" comes after "jesam/jesi/je/jesmo/jeste/jesu li".

First 1a and 1b are correct