Ég man það núna, en ég gleymi því stundum.

Because Icelandic verbs don’t use the infinitive in finite clauses. The verb muna is irregular in the present tense; the 1st person singular is ég man. Key present forms:

• ég man
• þú manst
• hann/hún/það man
• við munum
• þið munið
• þau/þeir/þær muna

Don’t confuse muna “remember” with the auxiliary munu “will.” For example, við munum can mean either “we remember” (from muna) or “we will” (from munu); context disambiguates.

Past of muna is mundi (with u): Ég mundi það “I remembered it.” That’s different from myndi (with y), which is the conditional of munu (“I would”).

Different verbs take different cases:

• muna e-ð “remember something” takes the accusative, so the pronoun is það.
• gleyma e-u “forget something” takes the dative, so the pronoun is því.

Relevant forms of “it”:

• nominative/accusative: það
• dative: því
• genitive: þess

You can drop the object if you mean it generally:

• Ég man (núna). = “I remember (now).”
• Ég gleymi stundum. = “I forget sometimes.”

But if you’re referring to a specific thing previously mentioned, you normally keep the object pronoun:

• Ég man það núna.
• Ég gleymi því stundum.

If the “it” is an entire clause, use að:

• Ég man að hann kom. = “I remember that he came.” (No það needed.)

Time adverbs like núna often come after the object: Ég man það núna. You can also front it; Icelandic main clauses are verb-second (V2), so the verb still comes second:

• Núna man ég það.

All of these are acceptable, with small differences in emphasis:

• Ég gleymi því stundum. (very natural; short pronoun early)
• Ég gleymi stundum því. (also fine)
• Stundum gleymi ég því. (fronting stundum for emphasis on “sometimes”)

En (“but”) is a coordinating conjunction. When it connects two independent clauses, Icelandic normally places a comma before it:

• Ég man það núna, en ég gleymi því stundum. If it only connects smaller phrases, the comma may be omitted.

Typical placements:

• “I don’t remember it now”: Ég man það ekki núna. / Núna man ég það ekki.
• “I don’t forget it anymore”: Ég gleymi því ekki lengur. In simple main clauses, the finite verb is in second position; ekki usually follows objects/complements.

Present:

• ég gleymi, þú gleymir, hann/hún/það gleymir
• við gleymum, þið gleymið, þau/þeir/þær gleyma

Past:

• ég gleymdi, þú gleymdir, hann/hún/það gleymdi
• við gleymdum, þið gleymduð, þau/þeir/þær gleymdu

Past participle: gleymt. The verb always takes the dative object: gleymi því.

Both are fine:

• With repetition (very common and clear): Ég man það núna, en ég gleymi því stundum.
• Without repetition (also natural in coordination when the subject is the same): Ég man það núna, en gleymi því stundum.
  Icelandic isn’t generally a “drop-subject” language, but coordinated clauses often omit a repeated subject.

Use the pronoun that matches the antecedent’s grammatical gender and case:

• Masculine: hann → acc. hann, dat. honum (e.g., bíll “car”): Ég man hann / Ég gleymi honum.
• Feminine: hún → acc. hana, dat. henni (e.g., bók “book”): Ég man hana / Ég gleymi henni.
• Neuter: það → acc. það, dat. því (e.g., mál “matter”): Ég man það / Ég gleymi því.

When “it” refers to a clause or abstract “that,” það is common, or you switch to að + clause.

Yes. Fronting is common for emphasis, and the verb still stays in second position:

• Stundum gleymi ég því.