Learn German: Es ist zweifelhaft, ob er das Design gestohlen hat. - It is doubtful whether he stole the design.

The suffix -haft attaches to the noun der Zweifel (the doubt) to create an adjective. It generally means 'having the nature of' or 'full of,' similar to the English suffix '-ful.' Therefore, zweifelhaft translates to 'doubtful' or 'dubious.'

Could I use wenn instead of ob in this sentence?

No, you must use ob. While English uses 'if' for both conditions and indirect questions, German strictly separates them. ob means 'whether' and is used when there is a yes/no uncertainty, like wondering whether he stole the design or not. wenn is only used for conditional 'if' statements or temporal 'whenever' statements.

Why does the helping verb hat come at the very end, after the past participle gestohlen?

The word ob is a subordinating conjunction that introduces a dependent clause. In German dependent clauses, the conjugated verb (here, hat) must be pushed to the very end of the clause, which places it after the past participle gestohlen.

Is gestohlen formed from a regular or irregular verb?