Learn Italian: Non sono sicuro che queste leve seguano una sequenza. - I am not sure that these levers follow a sequence.

The phrase Non sono sicuro che (I am not sure that) is a negative declaration of certainty. Because it introduces doubt and uncertainty into the statement, it triggers the subjunctive mood for the verb in the following clause.

If the sentence were affirmative ("I am sure that..."), would I still use the subjunctive?

No. If you drop the Non and say Sono sicuro che, you are declaring certainty. Certainty takes the indicative mood, so the sentence would change to: Sono sicuro che queste leve seguono una sequenza.

Seguire is an -ire verb. Why does seguano have an -a in the ending?

In the present subjunctive, regular -ere and -ire verbs take endings with the vowel -a (rather than the -i or -o you see in the present indicative). For the "they" (loro) form, this results in the -ano ending, giving us seguano.

How would I say "I'm not sure that I follow a sequence" using this structure?