Learn Hardcore Swahili: Wikiendi hii, sisi tutacheza mpira na marafiki uwanjani. - This weekend, we will play ball with friends in the field.

Wikiendi is a borrowing from English weekend.
hii is a demonstrative meaning this, and it agrees with wikiendi (which is in noun class 9/10). In Swahili, demonstratives change form to match the noun class of the noun they modify.

Why is sisi included in the sentence? Isn’t the subject clear from the verb?

In Swahili, the subject prefix on the verb (in tutacheza, tu- indicates “we”) already tells you the subject.
sisi is the independent pronoun we, added for emphasis or clarity. You can omit it and still say Tutacheza mpira... meaning the same thing.

How is the future tense formed in tutacheza?

The future tense formula is:

Subject prefix (we = tu-)
Tense marker (-ta- for future)
Verb root (-cheza “play”)
So tu- + -ta- + cheza = tutacheza, “we will play.”

What does mpira mean?

mpira literally means ball, but in context it often refers to football / soccer, since that’s the most common ball game in many Swahili‐speaking regions.

Why is na used in na marafiki and what does it mean here?

na can function both as and (connecting nouns) and as with (indicating accompaniment).
In tutacheza mpira na marafiki, na means with, so it reads “play ball with friends.”

What is uwanjani, and how is it formed?

uwanja means field or open ground.
To express location (“on the field”), you add the locative suffix -ni to get uwanjani.

Is the word order in Swahili always Subject–Verb–Object–Location?

The default order is S–Tense–V–O–Locative, as in Sisi tutacheza mpira uwanjani.
You can move Wikiendi hii to the front for emphasis/time reference, yielding Wikiendi hii, sisi tutacheza mpira na marafiki uwanjani.