When in Rome...

The similarities between Latin and Modern Spanish (as well as other Latin-derived languages) are stunning. Herein I will focus primarily on Spanish, but where I feel fitting, I shall impart my knowledge of shared traits of sister tongues.

LATIN "to be" - esse
Sum / Sumus
Es / Estis
Est / Sunt

ITALIAN "to be" - essere
Sono / Siamo
Sei / Siete
È / Sono

SPANISH "to be" - ser
Soy / Somos
Eres / Sois
Es / Son

BRAZILIAN PORTUGUESE "to be" - ser
Sou / Somos
És / Sois
É / São

Following with pronouns, most comparisons will be between Spanish and Latin, but I may even include pronouns of pronouns of distant languages.

Latin Pronominal System
Nominative
Ego / Nos
Tu / Vos

Accusative
Me / Nos
Te / Vos

Spanish Pronominal System
Nominative
Yo / Nosotros (Nosotras)
Tú / Vosotros (Vosotras)

Accusative
Me / Nos
Te / Vos

In Greek, the first person nominative pronoun, like Latin, is also Ego. The third person pronouns (he, she, it, they) are omitted because there are no similarities between them (though, if you have ever taken a psychology class or are at least familiar with Freud's concept of the Ego and the Id, id is the third person nominative singular equivalent of the English it), since Latin uses either adverbs of location (here, there, over there) or calls the person by a major characteristic respectively (i.e. saying Pater, "Father," instead of "He").

Portuguese's Pronominal System
Nominative
Eu / Nós
Tu / Vocês

Accusative
Me / Nos
Te / Os or As

The pronoun vocês is the plural form of você, the polite second person form of address (informal equivalent is tu in Portuguese), like Spanish's usted (which is actually a contraction of vuestra merced, "your mercy" or "your majesty," that was used as a form of polite address to a stranger or a person whose social status called for it in the 16th century, and also explains why it may be abbreviated to Ud. or Vd.) and the plural ustedes (or vuestras mercedes). There is a vós and vos form, but it is archaic now.

Russian's Pronominal System
Nominative
Я (Ya) / Мы (Mi)
Ты (Ti) / Вы (Vi)

Animate Accusative and Genitive
Меня (Menya) / Нас (Nas)
Тебя (Tebya) / Вас (Vas)

I suspect, keeping in mind that I am only an amateur, however, that the pronouns in these languages probably came from a previous common language, the supposed Proto-Indo-European language. I will probably research it more indepth when I have more reliable sources available to me.

The pronouns in the Latin Genitive Pronominal System act as adjectives since they modify the noun to show possession and therefore are inflected to show gender and number (like Spanish) and case (like Russian).

Compare:

LATIN
Meus (Me-) / Noster (Nostr-)
Tuus (Tu-) / Vester (Vestr-)

SPANISH
Mi or Mío / Nuestro
Tu or Tuyo / Vuestro

The position of these pronouns in Latin in a sentence are the same as in Spanish and even Italian, but rules differ in Portuguese and Russian. You might say to your husband or wife (if you are married) in English I love you, while in Latin you would say Ego te amo ("I you love") or Te amo, since the verbal inflection already shows the person doing the action as "I," and thus using Ego would place contrast or emphasis on the subject (Your enemies hate you, but I love you). The Spanish equivalent follows the same path with Yo te amo, but Te amo will do just fine. Italian joins the crowd, too, using Io ti amo or simply Ti amo. Brazilian Portuguese, on the other hand, places the accusative pronoun te before the verb amo if the nominative pronoun eu is present to yield Eu te amo. Otherwise, it becomes Amo-te. In European Portuguese, the accusative pronoun is suffixed to the verb, nominative pronoun or not: Eu amo-te or Amo-te. The only exception to the rule is an interrogative pronoun, an adverb of negation or any other adverb, or a preposition before the verb.

Finally, Russian usually prefers a fixed Subject-Object-Verb word order, as in Я тебя люблю (Ya tebya lyublyu, "I you love"), but it may also be Я люблю тебя (Ya lyublyu tebya, "I love you"), Тебя я люблю (Tebya ya lyublyu, "You I love"), Люблю тебя я (Lyublyu tebya ya, "Love you I"), Люблю я тебя (Lyublyu ya tebya, "Love I you"), or Тебя люблю я (Tebya lyublyu ya, "You love I").

This is only a preview of what is to come in the future.