היום אני קצת חלש, ולכן אני לא נשאר בעבודה עד מאוחר.

Yes—historically, היום is literally the day, but in modern Hebrew it very commonly means today.

Context tells you which meaning is intended:

• היום אני קצת חלש = Today I’m a little weak
• היום שבו... = the day when...

At the beginning of this sentence, היום is naturally understood as today.

Because Hebrew normally does not use a present-tense form of to be.

So:

• אני חלש = I am weak
• אני קצת חלש = I am a little weak

This is completely normal Hebrew.
In the past or future, Hebrew does use forms of היה:

• הייתי חלש = I was weak
• אהיה חלש = I will be weak

Because חלש is the masculine singular form.

Hebrew adjectives agree with the person or thing they describe in gender and number. Since the speaker here is masculine singular, the sentence uses:

• חלש = masculine singular

If the speaker were female, it would be:

• היום אני קצת חלשה, ולכן אני לא נשארת בעבודה עד מאוחר.

Notice that both the adjective and the present-tense verb change:

• חלש → חלשה
• נשאר → נשארת

קצת means a little, a bit, or sometimes kind of / somewhat, depending on context.

Here:

• אני קצת חלש = I’m a little weak

It softens the statement. It sounds less strong than simply saying:

• אני חלש = I’m weak

Placing קצת before the adjective is very natural.

ולכן means and therefore, so, or that’s why.

It is made of:

• ו־ = and
• לכן = therefore / so

Hebrew often attaches short prefixes like ו־ directly to the next word, so it is written as one word:

• ולכן

This connector is a bit more formal or structured than a simple אז (so), but it is very common and natural.

The dictionary form is להישאר, which means to stay or to remain.

So:

• להישאר = to stay / remain
• נשאר = staying / stays / am staying / stay (masculine singular present form)

Some common present-tense forms are:

• נשאר = masculine singular
• נשארת = feminine singular
• נשארים = masculine plural
• נשארות = feminine plural

It could sometimes be left out, but repeating אני is very natural here.

A key reason is that in the Hebrew present tense, the verb form usually shows gender and number, but not clearly person the way English does. So נשאר by itself does not automatically mean I stay—it could also fit he stays, depending on context.

Because of that, Hebrew often keeps the subject pronoun explicit:

• ולכן אני לא נשאר... = and therefore I’m not staying...

This makes the sentence clear and natural.

Good question. Hebrew often uses the present tense for something that is happening now, is a current plan, or is true for the situation at hand.

So:

• היום אני לא נשאר בעבודה עד מאוחר
  = Today I’m not staying late at work

This is very natural because it describes today’s plan/current decision.

You could also say:

• היום לא אשאר בעבודה עד מאוחר

That is also correct, and it sounds a bit more explicitly future or decisive.
So both are possible; the present tense here is not a mistake.

בעבודה means at work or in work, depending on context. In this sentence, it means at work.

The prefix ב־ means in / at.

One tricky point: in unpointed Hebrew spelling, בעבודה can represent slightly different vowel patterns, so learners often try to analyze whether it is:

• ב + עבודה or
• ב + ה + עבודה

In normal reading, you usually do not need to worry much about that. Here the natural meaning is simply:

• בעבודה = at work

Literally, עד מאוחר means until late.

Hebrew often uses this expression where English would say stay late:

• להישאר בעבודה עד מאוחר = to stay at work until late / to stay late at work

So the Hebrew phrase is built a little differently from the English one, but it is very idiomatic and common.

Not exactly.

• חלש means weak
• עייף means tired

In context, חלש can suggest that the speaker is feeling physically low, weak, or not at full strength. That may overlap with feeling tired, but it is not the same word.

So this sentence specifically says the speaker feels a little weak, not necessarily just sleepy or tired.

Yes. A few natural alternatives are:

• היום אני קצת חלש, אז אני לא נשאר בעבודה עד מאוחר.
  More conversational; אז = so

• היום אני קצת חלש, לכן אני לא נשאר בעבודה עד מאוחר.
  Similar meaning, slightly less connected than ולכן

• היום אני קצת חלש, אז לא אשאר בעבודה עד מאוחר.
  Uses the future tense instead of the present

All of these are reasonable. The original sentence is clear, natural, and well-formed.