אם לא נסכם את הפגישה עכשיו, מחר כבר נשכח מה היה הנושא הכי חשוב.

אם means if and introduces a condition.

So אם לא נסכם את הפגישה עכשיו means if we don’t summarize the meeting now.
This is very normal Hebrew word order: if + not + verb + object + time expression.

Hebrew commonly uses the future form after אם when talking about a real future possibility.

So:

• אם לא נסכם... = if we don’t summarize...
• literally, the verb form is future: if we will not summarize

This is different from English style, but it is standard Hebrew grammar.

נסכם is we will summarize / we summarize depending on context.
After אם, it naturally gets the meaning if we summarize / if we don’t summarize.

נסכם is the 1st person plural future form of לסכם.

• infinitive: לסכם = to summarize / to sum up
• future, we: נסכם = we will summarize

The root is ס־כ־ם.

A useful thing to know: לסכם can sometimes mean to agree on or to settle, depending on context.
Here, because the object is את הפגישה (the meeting), it means to summarize / wrap up the meeting.

את marks a definite direct object.

So in:

• נסכם את הפגישה

the word הפגישה is the direct object, and it is definite because of ה־ (the meeting), so Hebrew uses את.

Important points:

• את usually is not translated
• it appears before definite direct objects
• it does not mean with here

Compare:

• נסכם פגישה = summarize a meeting
• נסכם את הפגישה = summarize the meeting

הפגישה means the meeting, while פגישה means a meeting / meeting.

The prefix ה־ is the Hebrew definite article, like the in English.

So:

• פגישה = a meeting
• הפגישה = the meeting

Since the speaker is referring to a specific meeting that both people know about, the definite form is used.

מחר means tomorrow, and כבר often means already.

In this sentence, כבר adds a sense of by then, already by tomorrow, or as soon as tomorrow. It makes the warning stronger:

• מחר נשכח = tomorrow we’ll forget
• מחר כבר נשכח = by tomorrow we’ll already have forgotten / tomorrow already we’ll forget

This use of כבר is very common in spoken and written Hebrew when the speaker wants to stress how soon something will happen.

נשכח is the 1st person plural future of לשכוח (to forget).

• infinitive: לשכוח
• future, we: נשכח = we will forget

The root is ש־כ־ח.

So:

• מחר כבר נשכח = by tomorrow we’ll already forget / have forgotten

Because מה היה הנושא הכי חשוב is itself the thing being forgotten.

In English, we might say:

• we’ll forget what the most important topic was

Hebrew does the same directly:

• נשכח מה היה הנושא הכי חשוב

The clause beginning with מה acts as the object/content of forget.
So there is no need for an extra it.

You could think of it as:

• we’ll forget [what the most important topic was]

This is an embedded question, or content clause:

• מה = what
• היה = was
• הנושא הכי חשוב = the most important topic

Together: what the most important topic was

Inside the clause, Hebrew often keeps a word order that feels close to what was the most important topic.
After verbs like know, remember, forget, and understand, Hebrew commonly uses this kind of מה / מי / איפה / למה clause.

So:

• נשכח מה היה... = we’ll forget what was...

היה is the past tense form was (masculine singular).

It matches הנושא, which is a masculine singular noun:

• הנושא = the topic / the subject

So:

• מה היה הנושא... = what was the topic...

If the noun were feminine singular, you would expect הייתה instead.
For example:

• מה הייתה הבעיה? = what was the problem?

In modern Hebrew, the most common way to say most + adjective is:

• הכי + adjective

So:

• חשוב = important
• הכי חשוב = most important

Therefore:

• הנושא הכי חשוב = the most important topic

This is the usual everyday form.
A more formal alternative would be:

• הנושא החשוב ביותר

Both mean the same thing.

That is because the superlative pattern הכי + adjective behaves a little differently.

In ordinary adjective agreement, you often see definiteness on the adjective too:

• הנושא החשוב = the important topic

But with הכי, modern Hebrew normally says:

• הנושא הכי חשוב

not usually:

• הנושא הכי החשוב

So once הכי is used, the adjective usually appears without the article.

Yes. The sentence has a very natural Hebrew structure:

• אם לא נסכם את הפגישה עכשיו
  condition first: if we don’t summarize the meeting now
• מחר כבר נשכח מה היה הנושא הכי חשוב
  result second: by tomorrow we’ll already forget what the most important topic was

A few word-order features that may feel especially Hebrew:

• future form in both parts
• את before the definite object
• כבר placed after מחר for emphasis
• the embedded clause מה היה... used directly after נשכח

So even if some parts sound slightly unusual when translated word-for-word into English, the Hebrew sentence itself is very natural.