Learn Hardcore Swahili: Tembo zetu wanapumzika mtoni. - Our elephants are resting at the river.

wa- = 3rd-person plural subject prefix (“they”)
-na- = present-tense/continuous marker (“are …ing”)
pumzika = verb root meaning “rest”
So wanapumzika literally means “they are resting.”

How can I tell that tembo is plural when it looks the same in singular and plural?

Many Swahili nouns in class 9/10 (like tembo) have identical singular and plural forms. You recognize plural from agreement markers:

wa- on the verb (wanapumzika)
z- on the possessive (zetu)
Together they tell you tembo here = “elephants,” not “elephant.”

Why is the possessive zetu used, and why does it follow tembo?

Swahili uses possessive pronouns that agree with the noun class and always come after the noun. For class 9/10 nouns the 1st-person-plural possessive is zetu. Hence:

tembo zetu = “our elephants”

What is mtoni, and how is it related to mto (“river”)?

mtoni is the locative form (class 18) of mto (“river”). Formation:

Take the stem to from mto
Add the class 18 prefix m- and suffix -ni
Result m-to-ni meaning “at/in the river.”

Can I use katika mto instead of mtoni? Is there a difference?

Yes. katika mto (“in the river”) uses the preposition katika + noun. mtoni is shorter and idiomatic for “at/in the river.” Both are correct; mtoni simply employs the locative class.

How would I express this sentence in the past or future tense?

Swap the tense marker -na- for:

-li- = past tense → Tembo zetu walipumzika mtoni. (“Our elephants rested in the river.”)
-ta- = future tense → Tembo zetu watapumzika mtoni. (“Our elephants will rest in the river.”)

What if I want to say “our elephant is resting in the river” (singular)?

You still use tembo but change agreement to singular:

Possessive for class 5 (often used for a single large object) is le-tu
Subject prefix for 3rd-person singular class 5 is li-
Result:
Tembo letu linapumzika mtoni. = “Our elephant is resting in the river.”