אחרי האימון הראשון הייתי עייפה, אבל עכשיו כל אימון קל יותר.

Because עייפה is the feminine singular form of tired.

The sentence tells us the speaker is female:

• הייתי עייפה = I was tired said by a woman

If a man were speaking, it would be:

• הייתי עייף

A useful point: הייתי itself does not change for gender in the first person singular. The gender shows up in the adjective:

• הייתי עייף = I was tired. (male speaker)
• הייתי עייפה = I was tired. (female speaker)

הייתי means I was.

It comes from the verb להיות = to be.

In this sentence:

• אחרי האימון הראשון הייתי עייפה = After the first workout, I was tired

This is the past tense, first person singular.

A few related forms:

• הייתי = I was
• היית = you were (to a male / to a female, pronounced differently in careful speech but often similar in modern speech)
• היה = he was
• הייתה = she was

Because the phrase means the first workout, and in Hebrew, when a noun is definite and is followed by an adjective or ordinal number, both usually take ה־.

So:

• אימון ראשון = a first workout / first workout
• האימון הראשון = the first workout

Here:

• האימון = the workout
• הראשון = the first

This is very common in Hebrew:

• הספר הגדול = the big book
• היום הראשון = the first day
• האימון הראשון = the first workout

Because הראשון means first, while אחד means one.

So:

• האימון הראשון = the first workout
• אימון אחד = one workout

These are not the same idea:

• first = position in order
• one = quantity

So if you want to talk about the workout that came first in a series, you use:

• ראשון / ראשונה

Not:

• אחד / אחת

Because in Hebrew, in the present tense, the verb to be is usually not expressed.

So:

• כל אימון קל יותר literally looks like every workout easier more
• but it naturally means every workout is easier

This is completely normal Hebrew.

Compare:

• אני עייף / עייפה = I am tired
• הוא בבית = He is at home
• האימון קשה = The workout is hard

But in the past and future, Hebrew does use forms of to be when needed:

• הייתי עייפה = I was tired
• אהיה עייפה = I will be tired

Hebrew usually forms the comparative with:

adjective + יותר

So:

• קל = easy / light
• קל יותר = easier
• קשה יותר = harder
• מהיר יותר = faster

Unlike English, Hebrew usually does not add something like -er to the adjective. Instead, it adds יותר = more.

So:

• קל יותר = literally more easy
• natural English: easier

It is not stated here, because it is understood from the context.

The idea is:

• now every workout is easier
  meaning easier than before or easier than the first one

Hebrew often leaves than unstated if the comparison is obvious.

If you want to state it explicitly, Hebrew can use:

• מ־ or מאשר
• for example: קל יותר מהאימון הראשון = easier than the first workout

But in your sentence, that extra part is unnecessary because the comparison is clear already.

כל אימון means every workout or each workout.

A very important pattern:

• כל + singular noun

So:

• כל יום = every day
• כל פעם = every time
• כל אימון = every workout

Even though the meaning is repeated or general, the noun is usually singular in Hebrew.

Also notice that there is no ה־ here:

• כל אימון = every workout

Be careful not to confuse it with:

• כל האימון = the whole workout

That is a different meaning.

So:

• כל אימון קל יותר = every workout is easier
• כל האימון היה קשה = the whole workout was hard

אחרי means after.

In the sentence:

• אחרי האימון הראשון = after the first workout

It is followed by a noun phrase:

• אחרי האימון
• אחרי השיעור
• אחרי העבודה

It can also be used with pronoun endings in other contexts:

• אחריי = after me
• אחריך = after you
• אחריו = after him

But here it is simply the preposition after followed by the first workout.

Because Hebrew often places a time expression at the beginning of a sentence, just like English can.

So:

• אחרי האימון הראשון הייתי עייפה = After the first workout, I was tired

This fronted time phrase gives the setting first.

You could also rearrange it in some contexts:

• הייתי עייפה אחרי האימון הראשון

That would still be grammatical, though the original version is very natural because it emphasizes when the speaker was tired.

Not always.

אימון can mean:

• training
• practice
• workout

The exact translation depends on context.

In this sentence, workout is a very natural translation because of:

• the first workout
• every workout is easier

But in other contexts it could be:

• אימון צבאי = military training
• אימון כדורסל = basketball practice
• אימון אישי = personal training / coaching, depending on context

So אימון has a broader meaning than just one English word.

Here עכשיו literally means now, but like English now, it can refer to the current situation more generally.

So:

• אבל עכשיו כל אימון קל יותר = but now every workout is easier

This does not have to mean at this exact second. It means something like:

• at this point
• by now
• these days / currently

So the sentence contrasts:

• then: after the first workout, I was tired
• now: every workout is easier

Yes, קל is the masculine singular form, and it agrees normally with אימון, which is a masculine singular noun.

So:

• אימון = masculine singular
• קל = masculine singular
• כל אימון קל יותר = correct agreement

If the noun were feminine singular, the adjective would also be feminine:

• כל משימה קלה יותר = every task is easier

So there is nothing unusual here; it is standard adjective agreement.

Yes, but one word would change.

A male speaker would say:

• אחרי האימון הראשון הייתי עייף, אבל עכשיו כל אימון קל יותר.

A female speaker says:

• אחרי האימון הראשון הייתי עייפה, אבל עכשיו כל אימון קל יותר.

Everything else stays the same. Only the adjective tired changes to match the speaker’s gender.