שמתי מים, לחם ומעיל בתרמיל לפני שיצאנו ליער.

שמתי is the past tense, first person singular form of לשים (to put / to place).
So שמתי means I put.

A useful pattern: in many Hebrew past-tense verbs, the ending ־תי means I.

Because Hebrew usually does not need a separate subject pronoun when the verb already shows who did the action.

• שמתי already means I put
• יצאנו already means we went out / we left

You can say אני שמתי if you want emphasis, such as I put it, not someone else, but in a normal sentence it is usually omitted.

Hebrew uses את only before a definite direct object.

Here, מים, לחם ומעיל are understood as indefinite / nonspecific items, so there is no את.

Compare:

• שמתי מים, לחם ומעיל בתרמיל = I put water, bread, and a coat in the backpack
• שמתי את המים, את הלחם ואת המעיל בתרמיל = I put the water, the bread, and the coat in the backpack

The letter ו here means and.

In Hebrew, this conjunction is often pronounced ve-, but before certain sounds it changes pronunciation. Before a word like מעיל (me'il), it is pronounced u-, so ומעיל sounds like u-me'il.

So:

• ו = and
• ומעיל = and a coat

This is a normal pronunciation rule, not a different word.

בתרמיל is ב + תרמיל.

• תרמיל = backpack / knapsack
• ב־ = in / inside

So בתרמיל means in the backpack.

After verbs like put, Hebrew often uses ב where English may say in or sometimes into. So שמתי ... בתרמיל is a natural way to say I put ... in the backpack.

לפני means before.

When Hebrew says before followed by a whole clause, it often uses:

לפני ש־ = before ...

So:

• לפני שיצאנו = before we went out / before we left

You can think of ש־ here as introducing the following clause.

יצאנו is the past tense, first person plural form of לצאת (to go out / to leave).

So יצאנו means:

• we went out
• we left

The ending ־נו often marks we in the past tense.

Because in Hebrew, when certain prepositions attach to a noun with ה־ (the), the ה is absorbed.

Here:

• יער = forest
• היער = the forest
• ל + היער = ליער

So ליער means to the forest.

This also happens with ב and כ:

• ב + הבית → בבית = in the house
• כ + המלך → כמלך = like the king

In fully vowel-marked Hebrew, the pronunciation would show the definite article more clearly, but in normal spelling you just see ליער.

מים looks like a plural form, but it usually functions as a mass noun meaning water.

For a learner, the easiest way to treat it is simply as the normal Hebrew word for water. In this sentence, it is just one of the things being packed, so you do not need to worry much about agreement here.

No, Hebrew word order is somewhat flexible.

The given sentence is very natural:

שמתי מים, לחם ומעיל בתרמיל לפני שיצאנו ליער.

But you could also say:

לפני שיצאנו ליער, שמתי מים, לחם ומעיל בתרמיל.

That version emphasizes the time phrase before we left for the forest first. Both are normal.

Yes, literally יצאנו comes from לצאת, which means to go out or to leave.

So יצאנו ליער is literally something like we went out to the forest or we left for the forest.

In natural English, depending on context, you might translate it as:

• we left for the forest
• we went to the forest

The Hebrew keeps the sense of leaving / setting out.

Because Hebrew lists work much like English lists:

• items are separated by commas
• ו (and) usually appears before the final item

So:

מים, לחם ומעיל = water, bread, and a coat

That is the normal way to write a simple list in Hebrew.