Learn Italian: Non posso spiegare il segreto. - I cannot explain the secret.

In Italian, to make a sentence negative, you put non directly in front of the working (conjugated) verb. Here, that is posso. The whole phrase non posso acts together to mean 'I cannot' or 'I am not able to'.

Why is spiegare left in its full dictionary form (the infinitive)?

Because it comes right after posso, which is a modal verb (verbs like can, want, or must). When you use a modal verb, only the modal gets conjugated to match the subject (posso = I can). The action verb that follows it stays in the infinitive (spiegare = to explain).

Where is the word for 'I' in this sentence?

In Italian, you usually leave out subject pronouns like io (I) because the verb's form already tells you who is doing the action. The verb posso can only mean 'I can', so adding io is not necessary unless you want to add special emphasis.

Why does the sentence use il for 'the'?

Segreto is a masculine singular noun (which you can usually guess by the -o ending). Therefore, it needs the standard masculine singular word for 'the', which is il.