אחרי האימון אין לי כוח לעלות במדרגות.

In modern Hebrew, possession in the present tense is usually expressed with יש ל־ / אין ל־ rather than with a normal verb meaning have.

• יש לי כוח = I have strength / energy
• אין לי כוח = I don’t have strength / energy

Literally, אין לי כוח is something like there is not to me strength.

This is very common in Hebrew:

• יש לי זמן = I have time
• אין לי כסף = I don’t have money

So the sentence does not need a separate word for have.

Not exactly. אין לי כוח usually means:

• I don’t have the energy
• I’m too tired
• sometimes even I can’t be bothered

It is often more about energy, strength, or motivation than pure ability.

Compare:

• אני לא יכול לעלות במדרגות = I can’t go up the stairs
  (maybe physically impossible, or not allowed, or some other reason)
• אין לי כוח לעלות במדרגות = I don’t have the energy to go up the stairs

So this sentence sounds more like fatigue than inability in a strict sense.

כוח literally means strength, power, or force, but in everyday speech it is very often used in expressions about energy.

So אין לי כוח is a very common idiomatic phrase. In natural English, you would often translate it as:

• I don’t have the energy
• I’m too exhausted
• I just can’t deal with it right now

Even though כוח literally means strength/power, in this expression it behaves a lot like energy.

Because after כוח here, Hebrew uses the infinitive to express what the person has or doesn’t have the energy to do.

So:

• אין לי כוח לעלות = I don’t have the energy to go up
• יש לי כוח לעבוד = I have the energy to work
• אין לי כוח לדבר = I don’t have the energy to talk

The ל־ on לעלות is the normal infinitive marker, like to in English.

לעלות basically means to go up, to ascend, or to climb.

In this sentence, it means going upward by means of stairs, so in English you might say:

• to go up the stairs
• to climb the stairs

The root is connected with upward movement, and you’ll see it in many contexts:

• לעלות לקומה השנייה = to go up to the second floor
• לעלות להר = to climb a mountain

Here במדרגות literally means by stairs / via the stairs / on the stairs.

The noun is:

• מדרגה = step
• מדרגות = stairs / steps

And ב־ can mean several things depending on context, including in, on, with, or by means of.

So לעלות במדרגות is a natural way to say to go up by the stairs or to climb stairs.

This is slightly different from לעלות את המדרגות, which more directly means to climb the stairs with the stairs as a direct object.

Both can be understandable, but לעלות במדרגות is a very common and natural phrasing.

Because stairs in Hebrew, as in English, is usually expressed as a plural idea.

• מדרגה = one step
• מדרגות = stairs / steps

So even when English says the stairs as one unit, Hebrew also normally uses the plural מדרגות.

האימון means the workout / the training session.

Hebrew often uses the definite article ה־ when referring to a specific event already understood from context.

So:

• אחרי אימון = after a workout / after training
• אחרי האימון = after the workout / after the training session

Both are possible, but אחרי האימון sounds like a particular workout, not workouts in general.

אחרי means after.

So:

• אחרי האימון = after the workout

It introduces a time expression and tells us when the rest of the sentence is true.

You can use אחרי with many nouns:

• אחרי העבודה = after work
• אחרי השיעור = after class
• אחרי הארוחה = after the meal

Yes, the order could be different. Hebrew word order is somewhat flexible.

This sentence starts with אחרי האימון to set the time first:

• אחרי האימון אין לי כוח לעלות במדרגות.

You could also say:

• אין לי כוח לעלות במדרגות אחרי האימון.

Both are grammatical. The version with אחרי האימון at the beginning feels very natural because it frames the situation first: after the workout...

Because Hebrew does not need אני here.

The phrase אין לי already tells you who the experiencer is:

• לי = to me / for me

So אין לי כוח already means I don’t have energy.

Adding אני would usually be unnecessary unless you want extra emphasis:

• אני, אחרי האימון, אין לי כוח...
  This would sound marked or emphatic, not neutral.

Yes, it is understood as a present-time statement.

Hebrew uses יש / אין for present-time possession or existence:

• יש לי כוח = I have energy
• אין לי כוח = I don’t have energy

So the sentence means that right now, in the situation after the workout, the speaker does not have the energy to climb the stairs.

Yes, but the tone changes a little.

• אחרי is the everyday, common word for after
• לאחר is more formal or written

So:

• אחרי האימון אין לי כוח לעלות במדרגות = natural everyday Hebrew
• לאחר האימון אין לי כוח לעלות במדרגות = correct, but slightly more formal

In conversation, אחרי is usually the better choice.