Learn Arabic (Modern Standard): نلعب التنس بعد المطر.

بعد is a preposition of time. Just like the basic prepositions you have already learned (such as في or من), it causes the noun following it to take the genitive case. This means that المطر (the rain) technically ends with a 'kasra' (an 'i' sound) when spoken with full grammatical endings.

Where is the word for 'we' in this sentence?

The 'we' is built right into the verb! In the present tense, starting a verb with the letter ن (noon), as in نلعب (we play), tells us that 'we' are doing the action. You do not need to add the separate pronoun نحن (we) before it unless you want extra emphasis.

Why is there a 'the' (ال) attached to the word for tennis?

In Arabic, sports, games, and broad concepts almost always take the definite article ال (the). While in English we just say 'We play tennis', Arabic literally says 'We play the tennis' (نلعب التنس).

Why isn't the 'L' sound pronounced in التنس?

Even though it is a borrowed English word, it follows standard Arabic pronunciation rules. It starts with ت, which is a 'sun letter.' This means the 'L' sound of the ال (the) blends directly into the ت, making it sound like 'at-tanis' rather than 'al-tanis'.