בטח שכחת את המפתח בתיק השני; זה קורה לך לפעמים.

בטח is a very common spoken-word adverb. Here it adds the idea of a confident guess or assumption, like probably, surely, or I bet depending on tone and context.

So בטח שכחת... sounds natural and conversational, not especially formal. A more formal choice might be כנראה.

Also, this בטח is not the noun meaning security; it is a different use of the same spelling.

Because Hebrew usually does not need an explicit subject pronoun when the verb already shows who is doing the action.

שכחת already means you forgot in the past tense, so adding אתה or את is usually unnecessary unless you want extra emphasis or contrast.

That is very normal in Hebrew:

• שכחתי = I forgot
• שכחת = you forgot
• שכחנו = we forgot

In unpointed Hebrew writing, you often cannot tell from the spelling alone.

שכחת can represent:

• masculine singular you forgot
• feminine singular you forgot

The same thing happens later with לך, which can also be masculine or feminine in writing.

So this sentence, as written, can be said to either a man or a woman. In actual speech, pronunciation would normally make it clear.

את marks a definite direct object.

Since המפתח means the key, it is definite, so Hebrew uses את before it:

• שכחת את המפתח

This את does not mean you here. It is a grammatical marker with no direct English equivalent.

A useful rule:

• if the direct object is definite, Hebrew usually uses את
• if it is indefinite, Hebrew usually does not

For example:

• שכחת מפתח = you forgot a key
• שכחת את המפתח = you forgot the key

Because the preposition ב and the definite article ה combine.

So:

• ב + התיק becomes בתיק

In pointed Hebrew, the vowel would show the difference more clearly, but in ordinary unpointed writing you just see בתיק.

In this sentence, the whole phrase is clearly definite because of השני:

• בתיק השני = in the second bag / in the other bag

So even though you do not see a separate ה before תיק, the phrase is still definite.

Literally, השני means the second.

But in real usage, when there are two possible things, השני can also feel like the other one. So in context, בתיק השני may mean:

• in the second bag
• or naturally, in the other bag

Which shade is intended depends on the situation, not on the grammar.

Here זה works like English it in it happens.

It does not have to refer to some concrete object. It is just the normal way to build this kind of sentence:

• זה קורה = it happens
• זה קורה לי = it happens to me
• זה קורה לך = it happens to you

So זה is not especially emphatic here; it is just part of the standard expression.

Because the verb לקרות works with ל to mark the person affected by the event.

So Hebrew says:

• זה קורה לי = it happens to me
• זה קורה לך = it happens to you
• זה קרה להם = it happened to them

This is just the normal pattern of the verb. English uses to; Hebrew uses the prefix ל.

לפעמים is an adverb meaning sometimes. It tells you that this is not a one-time comment only; it is something that happens from time to time.

Its position is fairly flexible. For example, Hebrew can also say:

• לפעמים זה קורה לך
• זה לפעמים קורה לך
• זה קורה לך לפעמים

The version in your sentence, with לפעמים at the end, sounds very natural.

Because they do two different jobs:

• שכחת refers to one specific completed event: the forgetting
• זה קורה לך לפעמים gives a general comment about a repeated pattern

So the sentence first makes a guess about what happened, then adds a habitual observation. That mix of tenses is completely normal:

• past for the specific event
• present for the general truth or recurring situation

The semicolon links two closely related complete clauses:

• בטח שכחת את המפתח בתיק השני
• זה קורה לך לפעמים

It shows a stronger break than a comma, but not as final as a period.

In everyday Hebrew, many people might also write this with:

• a comma
• or a full stop

So the semicolon is fine, but it is more a punctuation choice than a special grammar rule.