Learn Arabic (Modern Standard): الولد يلعب في الثلج.

Arabic sentences usually start with the verb. However, putting the subject first (Subject-Verb-Object order) is a technique called "topicalization." Starting with الولد makes the boy the main topic of the sentence, adding a slight emphasis, like saying "As for the boy, he is playing in the snow."

Does the verb يلعب have to change if we change the subject?

Yes, the verb must match the subject in gender and number. Since الولد is masculine singular, we use the masculine يلعب. If the topic were "the girl" (البنت), the verb would change to the feminine تلعب.

Why is the "l" sound from the word "the" missing when pronouncing الثلج (ath-thalj)?

The letter ث is a "sun letter." When the word for "the" (ال) attaches to a word starting with a sun letter, the "l" sound blends into that letter. Instead of "al-thalj", you double the ث sound and say "ath-thalj".

How does the word "in" (في) affect the word for snow (الثلج) here?

في is a preposition, and in Arabic, prepositions change the grammatical case of the noun immediately following them. This puts الثلج in the genitive case, meaning if you pronounce the formal endings, it takes an "i" sound at the end: في الثلجِ (fi ath-thalji).