先日私がなくしたペンは新しいペンだった。

先日私がなくしたぺんは新しいぺんだった。

The pen which I lost yesterday was a new one.

先日(せんじつ)私(わたし)がなくしたペンは新(あたら)しいペンだった。

Senjitsu watashi ga nakushita pen wa atarashii pen datta.

先日 (せんじつ)

the other day; a few days ago

私 (わたし、わたくし)

I; me; private affairs; personal matter; secrecy; selfishness

が (が)

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

無くす (なくす)

to lose something; to get rid of; to lose someone (wife, child, etc.)

ペン (ペン)

pen

は (は)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

新しい (あたらしい)

new; novel; fresh; recent; latest; up-to-date; modern

だ (だ)

be; is; indicates past or completed action; indicates light imperative

Readings: セン、さき、ま.ず

Meanings: before, ahead, previous, future, precedence

Readings: ニチ、ジツ、ひ、 -び、 -か

Meanings: day, sun, Japan, counter for days

Readings: シ、わたくし、わたし

Meanings: private, I, me

Readings: シン、あたら.しい、あら.た、あら-、にい-

Meaning: new