הגרביים שלי רטובות, כי שכחתי את המטרייה בבית.

In the present tense, Hebrew usually does not use a separate verb meaning to be.

So:

• הגרביים שלי רטובות = My socks are wet
• literally, it is closer to My socks wet

This is completely normal in Hebrew.
If you wanted the past or future, then Hebrew would use forms of היה:

• הגרביים שלי היו רטובות = My socks were wet
• הגרביים שלי יהיו רטובות = My socks will be wet

Because in Hebrew, a noun with a possessive phrase like שלי is normally treated as definite.

So:

• הגרביים שלי = my socks
• literally: the socks of mine

This is standard Hebrew. The same pattern appears everywhere:

• הספר שלי = my book
• הבית שלה = her house
• החברים שלנו = our friends

English does not use the in this situation, but Hebrew usually does.

Hebrew usually expresses possession with:

• noun + של + pronoun

So:

• הגרביים שלי = my socks
• המטרייה שלך = your umbrella
• הילד שלהם = their child

English puts the possessive word first: my socks.
Hebrew usually puts it after the noun: the socks שלי.

There are some shorter possessive forms in Hebrew, but with everyday nouns, noun + שלי/שלך/שלו is the most common pattern.

Because adjectives in Hebrew must agree with the noun they describe in gender and number.

Here, גרביים is treated as a plural feminine noun, so the adjective must also be plural feminine:

• רטובה = wet, feminine singular
• רטובות = wet, feminine plural
• רטוב = wet, masculine singular
• רטובים = wet, masculine plural

So:

• הגרביים שלי רטובות = My socks are wet

If the noun were masculine plural, you would use רטובים instead.

The ending -יים often comes from the old Hebrew dual form, which was especially used for things that naturally come in pairs.

Examples:

• עיניים = eyes
• ידיים = hands
• נעליים = shoes
• גרביים = socks

In modern Hebrew, words with -יים usually behave like ordinary plural nouns, even if historically they were dual forms.

So you do not need to think of גרביים as some special separate number in everyday grammar. Just treat it as a plural noun.

שכחתי is a past-tense verb form.

It comes from the root ש־כ־ח, which has to do with forgetting, plus the ending -תי, which marks I in the past tense.

So:

• שכחתי = I forgot
• שכחת = you forgot (masculine singular)
• שכחתְ = you forgot (feminine singular)
• שכח = he forgot
• שכחה = she forgot

A very useful pattern to remember is:

• many past-tense I forms end in -תי

For example:

• אמרתי = I said
• כתבתי = I wrote
• שכחתי = I forgot

את is the Hebrew marker of a definite direct object.

It does not translate into English, but it tells you that the thing receiving the action is specific/definite.

So:

• שכחתי את המטרייה = I forgot the umbrella
• את marks המטרייה as the direct object

Compare:

• שכחתי מטרייה = I forgot an umbrella
• שכחתי את המטרייה = I forgot the umbrella

A good rule:

• use את before a definite direct object
• do not use it before an indefinite one

המטרייה means the umbrella. The prefix ה- is the definite article the.

So:

• מטרייה = an umbrella / umbrella
• המטרייה = the umbrella

In this sentence, the speaker is referring to a specific umbrella, so המטרייה is natural.

Hebrew often uses definiteness when the object is already understood from context. If the speaker simply meant any umbrella, they would more likely say:

• שכחתי מטרייה בבית = I forgot an umbrella at home

בבית is made from the preposition ב- plus בית.

• ב- = in / at
• בית = house / home

So בבית means:

• at home
• in the house

It is written as one word because Hebrew prepositions like ב, ל, and כ are often attached directly to the following word.

Also, when ב combines with the definite article ה, the form contracts:

• ב + הבית → בבית

In everyday unpointed Hebrew spelling, בבית can represent either in a house or in the house / at home; context tells you which meaning is intended.

Here, כי means because.

It introduces the reason:

• הגרביים שלי רטובות = My socks are wet
• כי שכחתי את המטרייה בבית = because I forgot the umbrella at home

So the whole sentence is:

• My socks are wet, because I forgot the umbrella at home.

Be aware that כי can also mean that in other sentences, depending on context. But in this sentence, because is the natural meaning.

Yes, it is completely natural.

The structure is:

• הגרביים שלי = subject
• רטובות = predicate adjective
• כי... = reason clause

So the sentence works as:

• My socks wet, because I forgot the umbrella at home

That may feel unusual to an English speaker because Hebrew:

• often has no present-tense is/are
• usually puts possession as noun + שלי
• marks adjective agreement clearly

But as Hebrew syntax goes, this sentence is very normal and idiomatic.