The conjunction dass introduces a subordinate clause that acts as a content clause, expressing a statement or thought. It is equivalent to the English word that when used to report something indirectly. For example, in the sentence Ich weiß, dass er kommt, the subordinate clause dass er kommt explains what is known, and it translates to “I know that he is coming.” The verb in the subordinate clause moves to the end, as is standard in German subordinate clauses.
In contrast, ob introduces a subordinate clause that functions as an indirect yes-no question. It is used when the subordinate clause expresses uncertainty between two possibilities, comparable to the English if or whether. In the sentence Ich weiß nicht, ob er kommt, the subordinate clause ob er kommt presents the two possibilities: either he is coming or he is not. The sentence means “I don't know if he is coming.”
The difference between dass and ob depends entirely on whether the subordinate clause reports a fact or introduces an indirect question. If the main clause refers to knowing, saying, hearing, or believing something specific, and the subordinate clause gives the content of that thought or statement, dass is used. For instance, Sie sagt, dass sie müde ist means “She says that she is tired.” But if the main clause introduces doubt, a question, or speculation about alternatives, ob is required. For example, Sie fragt, ob sie zu spät ist means “She asks if she is late.”
Some cases can seem ambiguous at first glance, particularly when verbs like wissen (to know) appear. When wissen introduces a subordinate clause with dass, it confirms knowledge of a fact. Ich weiß, dass er zu Hause ist means “I know that he is at home.” But when combined with ob, it reflects uncertainty or inquiry. Ich weiß nicht, ob er zu Hause ist means “I don't know if he is at home.” The presence or absence of negation in the main clause often signals which conjunction is appropriate, although the meaning is decisive: dass for facts, ob for alternatives.
Let's look at more complex examples. Er hat gehört, dass sie gesagt hat, dass das Konzert abgesagt wurde means “He heard that she said that the concert was canceled.” Both subordinate clauses report statements, so dass introduces each. On the other hand, Ich frage mich, ob er weiß, ob sie kommt means “I wonder if he knows if she is coming.” Here, the first subordinate clause introduced by ob contains another subordinate clause also introduced by ob, as both express indirect yes-no questions.
Sometimes, German allows alternative ways to express similar meanings without using dass or ob. Instead of Ich hoffe, dass du kommst (“I hope that you are coming”), one might simply say Ich hoffe, du kommst, omitting dass. In spoken language, this is common, though it is more typical in main clauses where the subordinate clause directly follows. However, such omission is not possible with ob because it indicates a question structure within the sentence. Ich weiß nicht, ob er kommt cannot drop ob without losing clarity.
Alternatively, infinitive constructions can sometimes replace dass-clauses. Instead of Ich hoffe, dass dich zu sehen (“I hope that I see you”), one might say Ich hoffe, dich zu sehen (“I hope to see you”). But this only works when the subject of both the main and subordinate clause is the same or clear from context. There is no such replacement for ob-clauses because they inherently involve alternatives that cannot be conveyed by an infinitive phrase.
Indirect yes-no questions introduced by ob can also sometimes be expressed through main clauses followed by direct questions, but this changes the sentence from indirect to direct speech. Instead of Ich weiß nicht, ob er kommt, one could say Kommt er? Ich weiß es nicht (“Is he coming? I don't know”), but the structure and tone shift significantly.