אחרי השיעור האחרון היה לי שיפור גדול בביטחון כשאני מדברת.

אחרי means after.

So אחרי השיעור האחרון = after the last lesson.

Yes, you could also use לאחר, which also means after, but it sounds a bit more formal or written. In everyday speech, אחרי is very common and natural.

In Hebrew, adjectives usually come after the noun.

So:

• שיעור = lesson
• אחרון = last/final

Together:

• השיעור האחרון = the last lesson

This is the normal Hebrew word order:

• noun + adjective

For example:

• ספר טוב = a good book
• הספר הטוב = the good book

Because when a noun is definite in Hebrew, its adjective usually has to be definite too.

So:

• שיעור אחרון = a last/final lesson
• השיעור האחרון = the last lesson

Both השיעור and האחרון take ה־ because the whole phrase is definite.

This is a very important Hebrew pattern:

• a noun + adjective
• if the noun is definite, the adjective is also definite

היה לי שיפור גדול literally means something like:

• there was for me a big improvement
• or more naturally, I had a big improvement

Hebrew often uses היה לי to express that someone had something.

So:

• היה לי = I had / there was for me
• שיפור גדול = a big improvement

This structure is very common in Hebrew.

For example:

• היה לי רעיון טוב = I had a good idea
• היה לי קושי = I had difficulty

So in your sentence, היה לי שיפור גדול means that the speaker experienced a major improvement.

Because שיפור is a masculine singular noun, and adjectives must agree with the noun in gender and number.

So:

• שיפור = masculine singular
• therefore גדול = masculine singular

If the noun were feminine, you would use גדולה instead.

Compare:

• שיפור גדול = a big improvement
• התקדמות גדולה = a big advancement/progress

בביטחון means in the confidence or more naturally here in confidence / in my confidence.

It is made from:

• ב־ = in
• הביטחון = the confidence

When ב־ attaches to a word with ה־, they combine:

• ב + ה → בַּ in pronunciation

But in spelling, because the noun itself starts with ב, you see two ב letters:

• בביטחון

It is pronounced roughly like ba-bitachon.

So the two ב letters are not a mistake:

• one belongs to the preposition ב־
• one belongs to the word ביטחון

Hebrew often leaves possession unstated when it is clear from context.

Here, the sentence already says היה לי = I had, so it is obvious that the improvement in confidence is the speaker’s own confidence.

So בביטחון can naturally be understood as:

• in confidence
• in my confidence

If you say בביטחון שלי, that is also possible, but it may sound a bit more explicit or emphatic.

So both are possible, but the shorter version is very natural when the meaning is obvious.

Because כשאני מדברת means when I speak or when I’m speaking, and it describes the situation in which the confidence appears.

So:

• כש־ = when
• אני מדברת = I speak / I am speaking

Together:

• כשאני מדברת = when I speak

If you used לדבר (to speak), the sentence structure would be different.

This clause makes the meaning more personal and specific:

• the speaker had more confidence when actually speaking

Because the verb agrees with the speaker’s gender.

• אני מדברת = I speak / I am speaking (female speaker)
• אני מדבר = I speak / I am speaking (male speaker)

So this sentence is being said by a woman or girl.

If a male speaker said it, it would be:

אחרי השיעור האחרון היה לי שיפור גדול בביטחון כשאני מדבר.

It can mean either, depending on context.

In this sentence, השיעור האחרון most likely means:

• the last lesson
• meaning the most recent lesson

But in some contexts, אחרון can also mean final.

For example:

• השיעור האחרון בקורס could mean the final lesson in the course
• but in casual conversation it can also simply mean the last one we had

So the exact nuance depends on context.

It is understandable, but many native speakers might say it a little differently.

Your sentence: אחרי השיעור האחרון היה לי שיפור גדול בביטחון כשאני מדברת.

Possible more natural alternatives:

• אחרי השיעור האחרון הרגשתי שיפור גדול בביטחון שלי כשאני מדברת.
• אחרי השיעור האחרון היה לי הרבה יותר ביטחון כשאני מדברת.
• אחרי השיעור האחרון הרגשתי הרבה יותר בטוחה כשדיברתי.

Why? Because שיפור גדול בביטחון is understandable, but Hebrew often prefers simpler, more direct phrasing such as היה לי יותר ביטחון = I had more confidence.

So the original sentence is not wrong, but it is slightly less idiomatic than some alternatives.