This is a good question, as it highlights how Hebrew, in its love of concision by cramming prepositions and possessive indicators into the prefixes and suffixes of verbs and nouns, can sometimes overload itself.
As you partly point out in your question:
- “you guarded” (m.sing.): shamárta
- “you guarded” (m.plural): shamártem
As you can see, the end of the verb determines the person form of the subject (i.e., the person doing the action). The problem arises when it has to do double duty to indicate the person of the direct object (the person that the action is done to), as well. It doesn’t always work. So while there is, for example:
- “you guarded me”: shmartáni (e.g. Job 10:14: חטאתי ושמרתני), and
- “you guarded us”: shmartánu (שמרתנו)
The former only appears once in the Hebrew Bible (in the very high-register—and probably archaic—Hebrew of the Book of Job), and the latter doesn’t appear there at all—only in the non-native Hebrew of Talmudic or early modern commentary (such as that of Metzudat Tzion, or in Sefer Ḥemdat Yamim).
If you push such constructions too far, there comes a point where the overloading can tip the whole thing over:
Which is why in the most definitive and classic example of the 3mpo (third-person-masc.-plural-object) contractions—Deut. 6:4–8 (which forms the basis of Tfilat Shma—probably the most important prayer in Judaism), the chosen suffix is simply -tam:
שמע, ישראל: יהוה אלוהינו, יהוה אחד.
ואהבת, את יהוה אלוהיך, בכל-לבבך ובכל-נפשך, ובכל-מאודך.
והיו הדברים האלה, אשר אנוכי מצווך היום–על-לבבך. ז ושיננתם לבניך, ודיברת בם, בשבתך בביתך ובלכתך בדרך, ובשוכבך ובקומך. ח וקשרתם לאות, על-ידך; והיו לטוטפות, בין עיניך.
—where, with regard to God’s commandments, we are told:
veshinantam levanekha […] uqshartam le’ot al-yadekha […]
(‘and you shall drill them in your children […] and you shall bind them as a mark on your arms […])
The result is a powerful metre, which underlines the imperative and insistent nature of the commandment:
veshinantam levanékha […] uqshartam le’ot al-yadékha […]
—so much so, that the –am suffix is carried through even to the indirect-object form in the verse: vedibárta bam (‘and ye shall talk about them’— instead of the usual bahem—which would have spoiled the metre).