Это была ловушка.

Это often means this or it in Russian. In sentences like Это была ловушка, it works a lot like English It was a trap or That was a trap.

Russian uses это very often in identifying statements:

• Это дом. = This is a house.
• Это был мой брат. = That was my brother.

So here, Это is the thing being identified as a trap.

Because ловушка is a feminine noun, and in the past tense, быть (to be) agrees in gender with the noun it refers to.

Past-tense forms:

• был — masculine
• была — feminine
• было — neuter
• были — plural

Since ловушка is feminine, Russian says:

• Это была ловушка.

Compare:

• Это был дом. — дом is masculine
• Это было письмо. — письмо is neuter
• Это были друзья. — друзья is plural

A very common clue is the ending. Nouns ending in -а or -я are often feminine.

So:

• ловушка ends in -а
• therefore it is usually feminine
• that is why the verb form is была

This is a very useful pattern, though there are some exceptions in Russian.

In Russian, the present-tense to be is usually omitted:

• Это ловушка. = This is a trap.

But in the past tense, Russian normally uses the past form:

• Это была ловушка. = It was a trap.

So:

• present: Это ловушка.
• past: Это была ловушка.
• future: Это будет ловушка.

Because this is a simple identification sentence: It was a trap. The noun naming what something was is normally in the nominative here.

So:

• ловушка is nominative singular

Older Russian and some formal styles can use the instrumental after forms of to be, but in a sentence like this, nominative is completely normal and standard.

Yes. Depending on context, Это была ловушка can be understood as:

• It was a trap
• That was a trap
• sometimes even This was a trap

Russian это is broader than English this in many sentences. The exact English translation depends on the situation.

Yes. Russian word order is more flexible than English, though the emphasis changes.

Common versions:

• Это была ловушка. — neutral
• Ловушка это была. — emphasizes trap
• Была ловушка. — less neutral, more contextual/literary
• Это ловушка была. — possible in speech, often contrastive

The most natural basic version is:

• Это была ловушка.

A simple pronunciation guide is:

EH-tuh by-LAH luh-VOOSH-kuh

A few notes:

• Это sounds roughly like EH-tuh
• была has stress on the last syllable: быЛА
• ловушка has stress on -ву-: лоВУшка

If you want the full stress-marked form:

• Э́то была́ лову́шка.

Russian has no articles like English a/an/the.

So ловушка can mean:

• a trap
• the trap

Context tells you which one sounds right in English.

That is why:

• Это была ловушка. can be translated as
• It was a trap or sometimes
• It was the trap
  depending on context, though a trap is the more natural translation in isolation.

The dictionary forms are:

• это — this / it / that
• быть — to be
• ловушка — trap

In the sentence:

• была is the feminine past-tense form of быть
• ловушка is already in its dictionary form: nominative singular

Yes. It is a fully natural, complete Russian sentence.

It can be used in many contexts, for example:

• after someone realizes they were deceived
• in a story
• in conversation
• in narration

For example:

• Мы пошли туда, но это была ловушка.
  We went there, but it was a trap.

So the sentence is short, standard, and very idiomatic.