zweitens
secondly
Used to introduce the second point in a list, an argument, or a speech.
Erstens ist es zu spät und zweitens bin ich zu müde.
Firstly, it's too late, and secondly, I'm too tired.

secondly
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secondly
Used to introduce the second point in a list, an argument, or a speech.
Erstens ist es zu spät und zweitens bin ich zu müde.
Firstly, it's too late, and secondly, I'm too tired.

'Zweitens' (secondly) belongs to a series of adverbs that structure a list: erstens, zweitens, drittens, viertens, etc. They are formed from ordinal numbers (erste, zweite, dritte) and the ending '-ens.'
Erstens habe ich keine Zeit, zweitens kein Geld und drittens keine Lust.
Firstly, I don't have time; secondly, I don't have money; and thirdly, I don't feel like it.
When 'zweitens' (secondly) stands at the beginning of a sentence, the conjugated verb comes directly after it, followed by the subject. This word order is called inversion.
Zweitens müssen wir die Kosten genau kalkulieren.
Secondly, we have to calculate the costs precisely.
These adverbs are very useful for structuring arguments or a list of points clearly and logically. They help the listener or reader follow the train of thought.
Warum ich nicht mitkomme? Erstens bin ich krank und zweitens regnet es.
Why am I not coming along? Firstly, I'm sick, and secondly, it's raining.
also / besides
'außerdem' and 'zweitens' both add further information. You use 'außerdem' when you simply mention another reason or another piece of information. There does not have to be an ordered list. You use 'zweitens' when you clearly number points: 'erstens, zweitens, drittens' (firstly, secondly, thirdly). Learners may confuse the words because both introduce another argument. If you only want to say something additional, use 'außerdem'. If you are making a sequence of arguments, 'zweitens' fits better.

Erstens ist der Termin zu früh, und zweitens habe ich an dem Tag keine Zeit.
Firstly, the appointment is too early, and secondly, I don't have time that day.
Ich nehme eine Jacke mit, außerdem soll es heute Abend regnen.
I'm taking a jacket with me; besides, it's supposed to rain this evening.