Můj syn hledá gumu a pravítko, protože je zapomněl v batohu.

Because můj has to agree with syn.

• syn is masculine singular
• it is the subject of the sentence, so it is in the nominative
• therefore the correct form is můj

So můj syn means my son, with můj matching the gender, number, and case of syn.

Both words are direct objects of hledá, so both are in the accusative.

But different noun types change differently:

• guma is a feminine noun, so its accusative singular is gumu
• pravítko is a neuter noun, and for many neuter nouns the nominative and accusative singular are the same, so it stays pravítko

So:

• guma → gumu
• pravítko → pravítko

Here guma means eraser.

In British English, rubber is also a natural translation. In other contexts, guma can mean rubber as a material, so the exact meaning depends on context. Since it appears with pravítko and school-related vocabulary, eraser is the intended meaning here.

In Czech, a subordinate clause introduced by words like protože is normally separated by a comma.

So:

• main clause: Můj syn hledá gumu a pravítko
• subordinate clause: protože je zapomněl v batohu

Czech punctuation is stricter here than English. In English, you would often write because without a comma before it, but in Czech the comma is expected.

Here je means them.

It refers back to gumu a pravítko. Instead of repeating both nouns, Czech uses the pronoun je:

• gumu a pravítko = the eraser and the ruler
• je = them

This is the accusative plural short form of the pronoun.

Because together they form a plural group: the eraser and the ruler.

Once Czech replaces that whole group with a pronoun, it uses the plural object pronoun je = them. You do not need separate pronouns for each noun.

So even though:

• guma is feminine
• pravítko is neuter

together they can still be referred to as je.

Because zapomněl agrees with the subject, not with the objects.

The subject is můj syn, which is:

• singular
• masculine

So the past-tense form is zapomněl.

The word je is only the object (them), and objects do not control the gender/number form of the past tense verb here.

Because short pronouns like je usually appear very early in the clause.

In Czech, unstressed pronouns often go in an early position, so:

• protože je zapomněl v batohu sounds natural
• protože zapomněl je v batohu sounds unnatural in normal speech

So this is largely a matter of normal Czech clitic placement: short pronouns like je tend to come near the beginning of the clause.

Because the preposition v meaning in takes the locative case when it expresses location.

Here the meaning is static location: he forgot them in the backpack.

So:

• dictionary form: batoh
• after v for location: v batohu

This is a very common pattern in Czech:

• v domě = in the house
• v autě = in the car
• v batohu = in the backpack

Because the sentence combines:

• what is happening now: hledá = is looking for
• what happened earlier: zapomněl = forgot

This is very natural in Czech. The son is currently searching for the items, and the reason is that he forgot them earlier in the backpack.

You can also notice aspect here:

• hledá is imperfective: an ongoing action
• zapomněl is perfective past: a completed event

That combination fits the meaning very well.