Перед пикником мы разложили плед на лужайке у воды.

Here is the breakdown:

• пикником — instrumental singular
  Used after перед.

• плед — accusative singular
  It is the direct object of разложили.

• лужайке — prepositional singular
  Used after на when talking about location.

• воды — genitive singular
  Used after у.

So the sentence is a nice example of how Russian marks relationships with cases instead of articles or a fixed word order.

It can mean either one.

• перед домом = in front of the house
• перед пикником = before the picnic

The context tells you which meaning is intended. Since пикник is an event, перед пикником is understood as before the picnic, not physically in front of it.

Because перед normally takes the instrumental case.

So:

• пикник → nominative
• пикником → instrumental

This is true both for the spatial meaning and the temporal meaning of перед:

• перед столом = in front of the table
• перед уроком = before class
• перед пикником = before the picnic

Both can mean before, but they are not always identical.

• перед пикником often means right before the picnic or in preparation for it
• до пикника is more like before the time of the picnic, often as a general time boundary

In this sentence, перед пикником sounds very natural because laying out a blanket is something you do as part of getting ready for the picnic.

Because плед is a masculine inanimate noun, and in the singular its accusative form is the same as its nominative form.

So:

• nominative: плед
• accusative: плед

Compare that with a masculine animate noun:

• я вижу брата = I see my brother

There the accusative does change. But for плед, it stays the same.

Because на лужайке describes location: where the blanket was spread out.

• на лужайке = on the lawn / on the grassy area
• на лужайку would suggest motion onto the lawn

A useful rule:

• на + prepositional = location
• на + accusative = direction/destination

So here the sentence focuses on the place where the blanket ended up being laid out.

у воды means by the water, near the water, or by the shore/edge of the water.

The preposition у takes the genitive case, so:

• вода → nominative
• воды → genitive

This is a very common pattern:

• у дома = by the house
• у окна = by the window
• у воды = by the water

It usually means near something, not necessarily touching it.

разложили is the past tense plural of the perfective verb разложить.

Here it means something like:

• laid out
• spread out
• arranged

The perfective aspect is used because the action is seen as completed: the blanket was laid out before the picnic.

Compare:

• разложили = they/we laid it out, completed
• раскладывали = they/we were laying it out, or used to lay it out

Also, for a blanket, a native speaker might very naturally say расстелили плед as well, meaning spread out the blanket.

It can be omitted if the context already makes the subject clear, but in this sentence мы is useful.

A key point: in the past tense, Russian verbs do not show person clearly the way present-tense verbs do.

• разложили can mean we laid out, they laid out, or you all laid out, depending on context.

So мы helps make it explicit: we laid out the blanket.

Yes. Russian word order is fairly flexible because the cases show what each noun is doing.

The given order is natural and neutral:

• Перед пикником мы разложили плед на лужайке у воды.

But you could also say:

• Мы разложили плед на лужайке у воды перед пикником.
• На лужайке у воды мы разложили плед перед пикником.

These versions are grammatically fine, but the emphasis changes a little. Russian often moves elements around to highlight time, place, or contrast.

плед is a masculine noun meaning a blanket, throw, or plaid rug/blanket.

In a picnic context, it is a very natural word for the blanket you put on the grass and sit on.

Its forms include:

• nominative: плед
• genitive: пледа
• dative: пледу
• instrumental: пледом
• prepositional: о пледе

So even though it may look a bit like a borrowed word, it declines in a normal masculine pattern.