"These girls are sisters."
Translation:Эти девочки — сёстры.
The Three Spelling rules (I do not expect anyone to memorize that, though)
There are some phonological constraints. If a noun ends in к/г/х or in ш/ж/щ/ч , only И is possible. Actually, it goes all the way to the core of the system: if you make ANY form of any word, and an Ы should follow one of these consonants, you use И instead.
If you are interested how come it works like that, it is due to the historical development of modern Russian
- ш, ж, ч (and щ, which was шч) used to be soft consonants, so И is a historical spelling
- ки, ги, хи actually come from old кы, гы, хы
- the old ки, ги, хи turned into чи, жи, ши, AFAIK
- so, кы, гы and хы have nowhere to come from. When using native Russian words, they only appear at word boundaries («мальчик и девочка»)
TL;DR stress happens to move in сестра. In Russian, stressed Е (historically) turned into Ё (but not everywhere)
- The stress of words may move depending on their form (the number of patterns is thankfully finite). Сестра is ending-stressed in the singular. In the plural, however, the initial syllable is stressed in all forms except the Genitive (сестёр).
- At some point over five hundred years ago stressed Е became Ё if a non-palatalised consonant followed it (e.g., Петр → Пётр, мед → мёд, меч → меч, день → день).
- So сестры, with a stress on the first е, becomes сёстры.
- It is messed up in the modern language. First, ш, ж, ц used to be palatalised but now are not. Ц was late to the party, so е remains unchanged if followed by ц (e.g., конец, отец). Second, Е is not the only Е. Russian also had a different vowel, something like /ɪe/ or /eː/ (spelt Ѣ). It merged with Е for the vast majority of dialects, but only a few centuries ago.
- So do not try to guess if you encounter an unfamiliar word. For words you know, though, either it turns into ё or it does not, in all forms that have the right stress (e.g., стена́→сте́ны, жена́→жёны)
Who knows? I would rather omit it in informal messaging in such a simple sentence. I tried my best to stick to standard punctuation in all my sentences and all other people's sentences that I had time to check. Still, I am not a writer, so in real life I get the commas and dashes mostly correct but definitely not 100% of the time.
Note that if one of these is a pronoun (not a noun!) or if you have a negation (let alone a question) rather than a declarative statement, no dash is required. Questions will almost never use it even for the sake of emphasis.
Singular neutral - это Sing. feminine - эта Sing. masculine - этот Plural - эти
If you use it directly with the noun, you have use the matching form: this girl - эта девочка, this boy - этот мальчик, this lake - это озеро, these girls - эти девочки
If you point at something, you always use это: This is a girl - это девочка
As you can see, since there is no word for "is", the right use of это is nescessary to get the difference between for example "this girl" and "this is a girl"