בלילה שמעתי רעם חזק וראיתי ברק מהחלון.

Hebrew often attaches short prepositions directly to the following word. Here, ב־ means in / at, so בלילה is one written unit.

Also, with very common time expressions, this whole form is just the normal way to say at night.

With the prepositions ב־, ל־, and כ־, the definite article ה־ usually does not stay as a separate letter. It gets absorbed into the form.

So ב + הלילה becomes בלילה, pronounced roughly ba-layla. In unpointed Hebrew, you just see בלילה.

Because the verb form already tells you the subject. The ending ־תי means I in the past tense.

So:

• שמעתי = I heard
• ראיתי = I saw

You can add אני for emphasis, but it is not necessary.

They are both past tense, first person singular.

• שמעתי comes from לשמוע = to hear
• ראיתי comes from לראות = to see

The ending ־תי is the key sign that the speaker is saying I did something in the past.

In Hebrew, ו־ means and, and it is usually written as a prefix attached to the next word.

So:

• וראיתי = and I saw

This is completely normal Hebrew spelling.

In Hebrew, adjectives usually come after the noun they describe.

So:

• רעם חזק = literally thunder strong
• natural English = loud thunder or strong thunder

That noun + adjective order is one of the basic word-order differences between Hebrew and English.

Because Hebrew often uses חזק for sounds in the sense of strong/intense, where English often prefers loud.

So רעם חזק is a natural Hebrew way to say loud thunder.
The same idea appears in phrases like מוזיקה חזקה for loud music.

Because את is used before a definite direct object. These objects are indefinite here.

So:

• שמעתי רעם חזק = I heard loud thunder
• ראיתי ברק = I saw lightning / a flash of lightning

If the speaker meant specific, known objects, you could say:

• שמעתי את הרעם החזק
• ראיתי את הברק

Yes. מ־ is the short prefixed form of מן, which means from.

So:

• מהחלון = from the window

More literally, it is מ־ + החלון. This is a very common pattern in Hebrew.

Literally, it means from the window, but in context English may translate it more naturally as through the window or out the window.

The idea is that the speaker saw the lightning by looking from the window area / out the window. If you wanted a more explicitly through meaning, Hebrew could also use דרך החלון.

A rough pronunciation guide:

• רעם ≈ ra-am or ra'am
• ראיתי ≈ ra-i-ti or ra'iti

In both words, the vowels are kept fairly separate. English speakers sometimes want to smooth them into one syllable, but in Hebrew they are clearer and more broken up.

Hebrew word order is fairly flexible. Starting with בלילה is very natural because it sets the scene first.

You could move it later, but that changes the rhythm and focus.
For example:

• בלילה שמעתי רעם חזק וראיתי ברק מהחלון = natural, scene-setting
• שמעתי רעם חזק וראיתי ברק מהחלון בלילה = possible, but less neutral in many contexts

So the original order is a very good, natural choice.