Once you start speaking in longer stretches, short disconnected sentences aren't enough — you need to link ideas: add one thought to another, present a choice, push back on something, give a reason, or correct a wrong statement. At the A1 level, German handles this with five essential connecting words: und (and), oder (or), aber (but), denn (because), sondern (but rather).
The great advantage of these five words is that they don't disturb the word order of the sentence that follows at all — the verb stays exactly where it normally goes, in second position, as if the connector weren't even there. Compare Ich lerne Deutsch with Ich wohne in Berlin, und ich lerne Deutsch: the verb lerne stays in second position in the second clause even though und was added before it. This is different from other connectors you'll meet at higher levels (like weil or dass), which push the verb all the way to the end of the clause — so enjoy this easier stage while it lasts.
The meanings are simple and direct: und joins two related ideas, oder presents a choice between alternatives, aber expresses a general contrast or contradiction, denn gives a reason for what was just said. sondern is the trickiest one, because while it also means something like "but rather," it can't be used as freely as aber.
The precise difference between aber and sondern is the heart of this lesson. aber always works for a contrast, whether the first clause is affirmative or negative, as in Ich bin müde, aber ich lerne (I'm tired, but I'm studying). sondern, on the other hand, only appears under one strict condition: the first clause must be negated with nicht or kein, and the second clause must correct that false information by replacing it with the right one — not just express a general contrast. Compare: Ich bin nicht müde, aber ich schlafe (I'm not tired, but I'm sleeping — a general contrast describing two separate facts) versus Ich bin nicht müde, sondern fit (I'm not tired, but rather fit — sondern here replaces one quality with the correct one within the same idea).
Many languages don't draw this line as sharply — a single word for "but/rather" often covers both cases — so learners frequently overuse aber everywhere, even where sondern is required, saying Ich bin nicht Student, aber Lehrer instead of the correct Ich bin nicht Student, sondern Lehrer (I'm not a student, but rather a teacher). The rule to hold onto: if the first clause is negative and the second clause corrects it rather than just contrasting with it, sondern is the only right choice.