Ef ég sef lengur í kvöld, þá verð ég hressari á morgun.

Þá literally means then. In conditional sentences it’s often used to mark the result clause: Ef X, þá Y = If X, then Y. It’s optional in many cases:

• Ef ég sef lengur í kvöld, þá verð ég hressari á morgun.
• Ef ég sef lengur í kvöld, verð ég hressari á morgun. Both are natural; þá adds a slightly clearer then-style connection.

Icelandic is a strong verb-second (V2) language in main clauses: the finite verb typically comes in the 2nd “slot.”
In the result clause, the first element is þá, so the verb comes next: þá + verð + ég.
Even without þá, starting a main clause after an Ef-clause usually triggers the same inversion:

• Ef …, verð ég … (not Ef …, ég verð … in standard style)

In Icelandic, as in English, the if-clause usually uses present tense to talk about the future:

• Ef ég sef … = If I sleep … (tonight)
  The future meaning comes from context (and from the result clause).

Verða basically means become, but it’s very commonly used to express a future/result state in conditionals and predictions:

• verða hressari = (to) become / end up (being) more refreshed So in English you often translate it as will be, even though the Icelandic idea is closer to will become.

Yes. munu + infinitive is another future-like option:

• … þá mun ég verða hressari á morgun. This can sound a bit more explicit or “forecast-like.” In everyday conditionals, simple verð is very common and natural.

Yes—hressari is the comparative of hress (refreshed / lively). Comparatives have limited agreement. Here hressari works for both masculine and feminine ég:

• Ég verð hressari. (speaker could be male or female) If the subject were neuter það, you’d normally use hressara:
• Það verður hressara.

These are set, idiomatic time expressions:

• í kvöld = tonight (literally “in evening-tonight”)
• á morgun = tomorrow You generally just learn them as fixed pairings. (Similarly: í dag = today, í gær = yesterday.)

It’s standard punctuation to separate an initial conditional clause from the main clause:

• Ef …, þá … You’ll often see a comma after an introductory subordinate clause in Icelandic writing.

Often yes, but it affects emphasis and must respect V2 word order in the main clause. For example:

• Ef ég sef lengur í kvöld, þá verð ég hressari á morgun. (neutral)
• Ef ég sef lengur í kvöld, þá verð ég á morgun hressari. (possible, a bit marked)
• Á morgun verð ég hressari ef ég sef lengur í kvöld. (also possible; main clause starts with Á morgun, then verb verð)

In an Ef-clause, you normally don’t use main-clause V2 the same way; a very common basic pattern is:

• Ef + subject + verb … → Ef ég sef … So it often looks more “English-like” inside that subordinate clause, while the following main clause shows the strong V2 behavior.